Hello there!

The guide here is based on my second trial on Endless Mode with my Co-Op partner Amarot, after we bashed through every level we found. Our first trial went pretty well, but unfortunately the last enemies overwhelmed us and we found ourselves defeated on the 99th% of our goal.
After this rather unnerving loss we decided to try out something, and what started as a joking try turned out to be quite possibly the dumbest way how to get far in this mode.

A partner makes this a lot easier, so grab a friend who is equally into getting Achievements and go at it.

You may take weapons you like, I personally took the Vampiric Gauntlets for the neat ability to Immobilize enemies. To be fair I used it half of the time to keep Moneybags in-game for that sweet 1.200C.

I would honestly say, take what suits you best. You can take up to three weapons, so feel free to take what you would like.

Warmage:

Vampiric Gauntlets:
Unique: Immobilizing the enemy.
To be honest, before this try of endless mode I never touched them. They are kind of neat to catch a stray kobold for their machinegun-like fire or just to endlessly stall a tanky ogre or troll as it has ludicrously low mana cost which means you always fire off a bolt per second to immobilize whatever you point at. It is possible to stall more than one, even though you should be careful about that.
Another good thing is the health to mana conversion; it can allow you to go to 1HP but reset your trinket or fire an powerful spell.

Ice Amulet
Unique: Enemies frozen by Secondary Attack take more damage.
Freezing that nasty troll in place to stall him a little or to let the thunder hit him is always neat, and it's 7 bolts are sure to wreak little havoc or keep orcs at bay. The secondary is more of a panic button and we combo'd it often with the thunderstorm of the Lightning Ring when it got tricky at the early stages.

Push Trap
Unique: Increased Range.
I picked this level specifically just so Pushers shine in their glory; even though later on you often see a single Kobold setting off every single one of them, allowing small fries to waddle into their doom a few steps later. They still do a lot, at least it felt like it.

Spore Mushrooms
Unique: Converted Kobold Runners become extremely dangerous.
THIS, my friend, is what you need. Early on to gnoll - and especially pesky kobolds - to ravage their own ranks brutally. Later on you will find them to be incredibly... useless. There the other ability shines: You make them useless! That giant mountain troll becomes shish kabob as the converted enemies get reset to their BASE values. You will see even armored ogres losing 1v1s against single Heavy Orcs.

Trap Reset Trinket
Just take both Specials. (Idiot me figured this out today that it wasn't uniques but specials.)
Did you wonder why you want to do this Co-Op? THIS. It resets the traps you have. And while it sadly doesn't reheat brimstone, it magically does magicify our magical mushrooms. IT. RESETS. MUSHROOMS. That's right. This is why it is a guide for dummies. Have a bit of coordination with your friend and you will always have your shrooms all magicked up.

Mana Rage Trinket
Spend those skulls comrade.
Mana. You need it, you want it, you get it. Refill your trap trinket, fire off that panic ice blast or whatever fancy thing you pull out of your infinitely large pockets to kill whatever threatens to ruin your achievement. Twice the trinket, twice the fun. And you still have the Vampiric Gauntlet blood bank in the back hand.


Sorceress:

Screptre of Domination
Unique: Charmed enemies take more damage.
A shroom to go, except it doesn't reset their values as their floor bound friends. The good thing: You get to choose which one you'd like... for the three seconds he survives. Also good to laser beam those kobolds. You might want to pick one up if you can.

Lightning Ring
Unique: Secondary attack applies Chilling effect to enemies.
Not too much to say, nice to roast a group of early orcs. Combos well with the aforementioned Ice Amulet. Also picks off frozen trolls and slows the early advances you face.

The map is Close Cliffs. Four pathes for four spawners, and one door, NOT a rift. That means no convenient regeneration.

The idea is that they are all very much on their own until they meet in the middle, right before the exit. While we decided to do the straight forward approach, one could also place barriers to create a tunnel. HOWEVER: Converted enemies are SURE to beat those things to a pulp while fighting the 'non-believers', so use at own risk!

At the first wave, have whoever takes Pushers with him place them at one wall to the fullest and the other five at the other wall of the upper spawners as shown above. His goal is later to plaster the rest of the wall and the two small walls where the lower spawners lead to; two for each side so you can get rid of small fries.

The other - if there is such a thing as jolly cooperation - can freely fiddle around with shrooms, placing them at the entrance of the wooden bridge or wherever you feel like.

At the start you will fend them off pretty manually.

To be honest. It is boring. But we wanted it to be boring, I mean we decided to just bring shrooms that we couldn't even eat ourselves.

The first few waves are just a brawl, the next waves will be a gladiator show of gnolls and kobold runners going ham on their comrades. Then, the power of converted enemies drops. RAPIDLY. That's when you find yourself just luring enemies into shrooms to make them pacified little idiots until they are picked off by their tenfold powerful cousins, just so they step into the next dozen of shrooms.

At this phase, you put your focus onto the few core threads and keeping your shrooms up. KEEP THEM UP! They are basically all beside you that are holding this mine.

After wave 50 done we decided to stand there and watch, apparently we have come through 6 more waves of doing nothing but punching Mr. Moneybags to buy more Skooma- erm, Shrooms.

These lords must die! In all honesty these enemies were annoying enough without shrooms but now they are THE thing you do not want in your questionably legal garden. They get converted, they split, they get converted again in an even more useless form... you get the idea. Up to 8 shrooms wasted for nothing, not even cannon fodder. Take them out as fast as you can.

As if they weren't annoying enough with insanely homing projectiles, they keep your mana regeneration at 0. And as you progress, their damage increases. I found myself dying to something and asked myself "What the hell killed me?!". Well, there's what it was. These annoying, one-eyed folks are sure to ruin any inattentive Warmage's day. You do your best stunning and mauling them to ashes.

Else, just coordinate when to burst those shrooms into life again to convert a mob of orcs.
NOTE: They have to walk onto it, so if they stand on it while you refill them, they will NOT trigger.

• When you use the trinket to reset shrooms, some might appear used up even though they are recharged, that's merely a graphic issue and no actual disadvantage; just check the bars whether they are up or not.
  
• As mentioned shortly above, the mushrooms trigger when an enemy steps into their border, e.g., steps onto it's outer edge.

Yeah we're gonna die, therefore I will give all my belongings to guppy, uhm wait- no.

This is it, I hope you enjoyed reading this, or found it to be helpful or informative in any way. That'd be really neat.

If you have any suggestions (MORE SHROOMS!) feel free to drop them here, same as just a simple 'thanks' or 'screw you', depending on how it worked out for you. Maybe it was the moustache? Who would know?

As a little bonus, here's a cheese maze of me and Amarot in Crunch Nightmare! (Take Health Regenerating Archers!)