Greetings.

The task of completing Rank 15 is a difficult one, but you will get needed skills while passing previous ranks, and, hopefully, some useful information from this guide.

Tactic is an art of commanding your army in battle.

1. Focus on firepower. At Rank 13 you’ll have +25 spread on your gun, and picking cards for shot distance is usually not going to help you, unless you’re very lucky to get cards to lower spread as well. For the most part of the game, you’ll have to shoot point-blank anyway.
2. Refill your pouch and reload all barrels, when it is safe. If not too many pieces left, hop around to refill your ammo, it will save you some irl time.

Kinda bad, but relatively safe. Can get you 1-2 shells.

3. Open all guns on rank 1, then pick one gun and master it. I personally prefer Victoria, because it can one-shot most pieces, and it will become a key part of your tactic.
4. Keep your shields on (can be adjusted in menu). You’re not protected from blunders, missclicks, etc., and losing because you accidentally step next to king on 11 floor is devastating. It should be noted, that sometimes shield will prevent you from making a shot you consider good if you’re in check, but it’s still better to keep them on.
5. Learn pieces move patterns. In general, all of them will attempt to get as close as possible to you to put you in check. Plan your moves in a way that will not let enemy do this on next turn. Knights are special in this case, because they can jump over pieces and therefore apply more pressure.

Red frames indicate immediate danger, black - squares that will be checked next turn.

They’ll also frequently team up. For everyone else you can predict their movement rather easily at the moment piece starts to wobble.

General idea of how pieces will act. Sometimes they change the pattern slightly, when in swarms. Notice how white squared bishop will attempt to get in close, even though it can't attack.

6. Don't let any type of pieces amass, especially bishops. Try to choose cards that won’t increase their number from the start. Bishops are the worst in this case, because they literally snipe you through the whole floor. Two bishops on each color usually are almost certain defeat. When possible, try to clear one color of tiles from them, and then they’ll pose almost no threat (although it may vary based on your cards).
7. Try to get close, while stayng safe. At the start of each floor move diagonally towards nearest (left or right) side of the floor. This way you usually won’t get in check by advancing bishops and knights at the start, winning you some turns to deal first damage.

This is fine. Might even one-shot him.

8. Try to maintain healthy number of pawns. Pawns block direct attacks from rooks and queen, hinder advance of other pieces and are relatively easy to kill, when getting to close to other side. You should largely ignore them until they cross 5th line (if we consider your starting line 8th, like in normal chess). Usually 6-8 pawns is okay, 8-10 with moat.
9. Cripple enemy's ability to check you. Between knight and bishops, I start with targeting knights, but as floor progresses and number of pawns gets lower, I switch to bishops. It is easier to avoid check from knights on empty field, and from bishops on swarmed.
10. Always target queen, when it’s possible to one-shot her. If she’s about to move, then make your move in a way to cause her to move in point-blank range. The exceptions are when you’ll either get multichecked or your firepower is too low.
11. Never trust Richard III to hit a target. I just really wanted to add this. At Rank 13 even Victoria barely hits at 3 tiles. Try to get as close as possible to your target, but also make sure to shoot from a safe spot, so that sudden miss wouldn’t mean sudden death.
12. Be mindful of where you shoot. Always keep an eye out for pieces, that have several other pieces blocking their line of attack. Calculate total amount of hp between you and them, so that when you shoot, you won’t accidentally break the wall and get checkmated. The same goes to grenades.

Good example. If I would by an chance hit pawn with 4 pellets, rook will instantly checkmate.

13. Try to point your gun in a way it would deal most possible damage. If you’re shooting piece point-blank, killing it is surely a priority, but any loose pellets are better to hit something too, especially if it’s a king, or any non-oneshotable piece. It will come in handy as floor progresses.

Numbers are their remaining hp. Potentially I can get 3 pawns with one shot. (Spoiler: I got one.)

14. Prioritize the most dangerous pieces. General target priority is Queen->Knights/Bishops->Rooks->King->Pawns. This is, of course, is for shots that will absolutely definitely hit: you shouldn’t try to snipe queen, when you have bishop in point-blank range and spread 100. Sometimes though, you should switch to targeting king directly (also at close range), if it is sure to bring quick end to the floor, and things could go awry otherwise.
15. Plan your turns in a way, that if you’re trapped, you can use soul to slither away.

Like this. (Right-click to cancel.)

Always try to keep maximum number of souls, if it’s not too dangerous. I personally prefer them in order: Queen->Bishop->Rook->Knight->Pawn (pawn souls are available only with Cannon Fodder card). Rook souls are not as effective, because most of pieces usually block straight lines movement, even bishops. Personally I’d also suggest to waste knight’s soul, if you get it at the start of first floor and try to replace it with bishop’s soul.
16. Always remain focused. Even two pieces can checkmate you. Also plan your shots and moves in a way, that you always can shoot enemy that gets close enough on next turn.
17. Boss is a dope. When you reach 12th floor, it will become a bit easier, because main boss is more of a distraction for a white army.

Strategy is an art of maintaining your country and army between battles, to get as great advantage as possible at hte start of next battle.

At some point of this game, you’ll start picking cards not by what good you want more, but by what bad you want less. Here I'll give my opinion on some cards and sometimes their conjunction with another cards.
1. The Moat and Deep Waters are the best possible cards. The only reason to discard them, is if white card would be devastating. Also don’t take The Bridge as a white card, because it’ll likely end you.
2. Pick Black Mist and Taunting Hop with high priority, also Royal Jousting, King's Shoulders, Magestic Censer, Posessed and Sacred Crown. Sometimes Bushido is also good, but only if it won’t prevent one-shotting. All of these cards give you extra turn or a chance to slither away/revive. Hopping is also useful to both reload and get in point-blank range to pieces sometimes. King's Shoulders won't let you relocate, but will let you pick up one piece and throw it away, if there's no other option.
3. Take cards that lower enemy hp amount, especially white cards Ruins and Undead armies. Golden Aging is also great, as it cripples queen's movement as floor progresses. The only possible exception is Knightmares, although even they are a better option in some cases. All these cards work great with Ravenous Rats and Black Plague, making your enemy pop in chain reaction. It should also be noted, that Rats can attack intangible Knightmares, and also they attack at the start of your turn, not the end, which means that is they clear the attack line of some white pieces and put you in check, that won't mean checkmate, and you still would have chance to get away. Ravenous Rats is one of the best cards overall.
4. Focus of firepower. Learn to use Sawed-off Justice and Blunderbuss. I personally also adore Cornered Despot. High Focus and Egotic Maelstorm are great cards, Holy Gunpowder is a bit more simple. Welcome Gift is ok, but works only once per floor. Piercing truth works well with all of them.
5. Presbyopia and Caltrops are great card for any field: first allows you to get in point-blank range to queens and bishops without any threat, while second reduces spd of all reinforcements and wounded pieces, giving you more precious turns to act.
6. Subtle poison works on every queen from reinforcements and promotions, and each has her own 15 turns timer, so it’s an incredibly strong card.
7. Small Fry Harvest, Force-Feeding, Extra Barrel, Ermine Belt (+3 shells) and Kingdom's Wealth (+6 shells +2 king hp) are all good. Rightful Curtsy even more, as it clears your way by pushing enemies sometimes.
8. Both Wand of Downpour and Wand of Wrath can be a useful way to get out of sticky situations, so don’t use wand charges unless you’re cornered. Wand of Downpour can also work in conjunction with Castle, as it might cause rook to move away. Wand of Gust, Wand of Hypnosis and Wand of Wings are also giving great sustain, especially because Wand of Wings lets you get through enemies. Wand of Frenzy is good, but it should be avoided if your ammo pouch is small.
9. Sniping is usually not an option, but if you get Engraved Scope (sniper)+Crow's Blessing/Elite Gem, with high damage you can get a nice shot on king through the whole field.
10. Ritual Dagger is pretty good card, but without Nightblade it won't give you much profit. But getting both of them with Bushido and Taunting Hop with will turn you into a murderous ninja.
11. Kingly Alms (grenades) are cool, but might backfire sometimes. It also can be upgraded with Philanthropy, giving you free throw (not free grenade) once per turn.
12. Undercover Mission, Imperial Shot Put, Cannonfodder and Unholy Call are largely situational cards and usually won't change floor progress a bit, although Undercover Mission might come in handy at the last turns. You can also use it to cancel Homecoming card (+1 queen) and send queen flying. Imperial Shot Put is pretty fun at lower ranks and blocks way a bit, but generally ill-advised to take in higher ranks. Cannon Fodder gives you +2 firepower on pawn's soul, but they occupy slots for more useful souls.
13. Royal Jester is a pretty good white card, because it can be upgraded with black card Fool Companion, that both gives you extra turn from killing jester and hinder white king’s movement.
14. Ammunition Depot, King's Mistress and Sanctity are pretty nice cards, because, although they add immediate reinforcements, they give ammo/queen range cut/not being much worse in return.
15. August Presence and Unfaithful Steed can be good sometimes, but I mostly don't use them.
16. I find Bloodless Coup, Unjust Decree, Cardinal Absolution and Church Organ mostly useless. You'll have to kill pawns at some point, so no Coup, and you have only one barrel, so you use Decree from the start. Cardinal Absolution will be inactive most of the time, unless you like to play with 0 backup plans. Church Organ is actually a very useful card by itself, but you need to have white card Cathedral, which is super bad. Also I didn’t really learn to use Royal Slippers properly (and shots always miss), so I’d suggest to avoid this card, unless you’re very good with it. There are usually better options.
17. Generally, reinforcements cards (Cavalry, Conclave, Lookout Tower) are not as bad as they look, unless combined with Conscription (spawn pawns). If you manage to clear most of the board before 15 turns, it won’t be much problem. It's a bit worse with Genderqueer card (queen reinforcements).
18. Saboteur and Shortage are kinda bearable cards.
19. Personally I dislike promotion cards Entitle and Cardinal, because they lower your max ammo count, and also Crusade and Peace for even slightly increasing amount of knights/bishops.
20. Divine Healing is bad, because it heals enemies, but sometimes useful, because bishops will always prioritize healing.
21. Secret Heir is not that bad, because most of the time you'll still have to clear most of pieces, but sometimes it can backfire dramatically. Last Guardian is similar, it won't give new king, but might bring queen instead.
22. Immediate reinforcements are pretty bad if not countered by heavy hp cut from other cards. Backups, Remparts, Succubus, Revolution, Pillage and Royal Court (sorted from least bad to the worst).
23. Assault, Zealots, Scouting, Red Book, Militia, Pikemen and Saddle add a lot of mobility and field control to white, and thus are very bad for you. Militia+Pikemen is a combination, that will likely end your game all by itself, same as Zealots+Red Book.
24. Karma and Witch's Curse are very bad white cards in my opinion, and they should be avoided, unless other option is even worse.
25. Avoid picking cards that increase enemy hp (Throne Room, Highest Dungeon, Bunker, Full Plate Armor and especially Trowel) and bring reinforcement periodically (Miltary Academy, Conscription). You might either get overwhelmed with a swarm or unkillable enemies. Or both.
26. Cathedral is a very bad card for you, because it really turns everyone around rooks into tanks. Same goes to Bunker.
27. Castle, Bodyguards and Kite Shield are very bad cards, because they force you to waste a lot of ammo on knights and rooks.
28. Ascension is one of the worst cards, because it turn bishops into flying terminators. Avoid it.
29. Theocracy is awful, because instead of one king you would have, like, five.
30. Analysis Paralysis and Guillotine are terrible cards: one of them makes you totally vulnerable for most precious first turns, while other will force you to kill all pieces. Avoid both.
31. Iron maiden is a no-go, because number of queens will rise, and your ability to kill them will not.

I had to shorten some terms, because I've reached symbol limit. I'm very sorry for that.
K-king, Q-queen, R-rook, B-bishop, N-Knight, P-pawn,
Damage-DMG, Firepower-FP, Fire Arc-FA, Ammo Max-AM, Speed-SPD

Personally I would put cards in this order, from the best to the worst:

I hope this guide will be helpful, but keep in mind, that it won't let you get victory from first try. You'll also require patience, persistence and a lot of luck. I wish you to get all the best combinations of cards when climbing ranks and good luck beating that white king on Rank 15!