To get the dreaded Abyssmal Shopaholic you need to unlock all items in the Abyss mode shop, then visit it one more time, this achievement took me a while to get thanks to the RNG, hopefully this guide can give you a few tips on how to get all the items you lack
For ease of completion, we are going to need an autotap program, any will do, I personally prefer Antimicro [sourceforge.net] to map the right analog up so it presses X 10 times a second, and map X to all 4 face buttons at the same time in the control configuraton so we can use a dirty trick to win very easily

If you don't use antimicro, investigating which software to use is up to you

My setup looks like this, in this example we use the X key to press ABCD at the same time
We press the button we want to config to the autofire (in this example, right analog up), then we are greeted by this screen
We select X in this screen's keyboard and then we press advanced below, There we adjust the delay to an amount we are comfortable with (I recommend at least 10 per second), and it should be all set, press close, then save the Antimicro preset

Lastly, we configure the keys in game and put ABCD to X for player 1 keyboard config, or whatever key you chose.

This extra mode adds a nice replayability to the game, but also it can be damn difficult when you start encountering unlimited bosses...

Every 100 floors we can leave the Abyss unlocking all new items we have found for purchase in the shop, and keeping the money we collected, but if we die in any stage, we will only unlock the items for the shop, and our money will be at the floor of the abyss in which we were defeated, so we can return and try to get the money back (just like the soul system in dark souls), but if we die before reaching the money, the first batch of money on a floor will dissapear.

Every 20 floors a new challenger will appear and our HP will be largely healed, after beating said boss we can choose 1 out of 5 options to improve our character, usually it consists of items 1 and 2 being a stat boost, item 3 an Ability or stat boost or depth changing item, 4 an Ability and 5 raw cash prize

Along the stat upgrades you might find a +X to All, as the name implies, is a small boost to all stats, its usually much weaker than what you will find to improve one specific stat

The attack stat increases your damage, Defense stat increases your HP, Speed stat increases your character speed when walking/running/dashing, and Heat stat sets your base heat at the start of a rebel to a certain ammount, and increases your heat gain.

There are a handful of equippable items to collect in the abyss, some more useful than others, and they usually come in 3 levels, unless they are a special item like a color or a passive like Free Barrier, which have no level, you may use any 3 items you want, as long as they aren't the exclusives enhancements of another character

Before going to the abyss you may visit the shop and buy items you have unlocked, which will cost us the in game currency of credits.

Abyss has 4 lenghts, 100 floors, 500 floors, 999 floors and Infinite, which has 99999 floors, thats not really infinite but that will take a loooong time to beat, you will probably die or get bored before completing them all.

Beating the 100 floors mode unlocks the 500 floors one, and beating that one unlocks the 999 floor mode, and beating this one unlocks the infinite floor mode

At floors 1000+ things get weird, you can get character enhancements from other characters that will do nothing to help you, but you still unlock them, also you get more powerful items, or maybe level 1 items, my guess is that the drops from floor 1000+ onwards can be anything.

There are special bosses in the Abyss at certain floors
Floor 400 has an Unlimited Hakumen
Floor 500 features an Unlimited Nu-13,
Floor 666 has an Unlimited Ragna with MAX SPEED AND ATTACK STATS!, this guy is incredibly fast and deadly, but he lacks HP, be extremly careful with this guy.
Floor 980 is where an Unlimited Hazama dwells, expect the same level of cheapness the Arcade mode boss Hazama has.
Floor 999 has an unlimited Mu-12 with maxed stats, expect this to destroy you, just like the hidden boss in arcade mode, and maybe even worse, her distortions hurt way too much, and if you drop the combo by even 1 frame she will do her reversal-like ability Tsunugui - Origins, which has full body guard point, meaning any hit she takes will be guarded instead of actually hitting her, it's okay to hate her

To go deeper into the Abyss, you need to do long combos to increase the depth, or hit certain thresholds of enemy HP left, around 1 depth for every 20% of their max health or so, but long combos work MUCH better for getting depth easily, always try to use your longest combos instead of hard hitting moves to get the proration stacking so you deal less damage and more hits if you want to advance effectively

To win easily almost every match in abyss mode we need 3 things

• Pick Arakune
  
• Become Unlimited ability
  
• Arakune Enhancement

To get Become Unlimited, you need to go deep into the abyss, around +800 and pray to the RNG gods that you get it after a boss battle, its pretty rare and there is almost nothing you can do to increase the chances of getting it, personally I got my Become Unlimited at stage 980 Boss Hazama, but getting to such stages of the abyss can be very hard for novice to intermediate players, for that I recommen either using your main character and pray you get a lucky drop, or simply spam the usage of Stylish Tager, just spam light for a combo, press Heavy for a magnetizing attack, and Special for his signature command grab, 6Special does his Sledgehammer special that makes you immune to bullets, with him it should be not very hard to get far into the abyss where the good drops are, and if we are in a bad situation or we are about to lose, we can always use the quicksave trick which we will discuss later in the guide

To get Arakune Enhancement, simply beat any abyss depth mode with Arakune, the 100 floor should be kids play for any character in stylish mode, even if you have no idea about how to use him/her

Then, when you have those 2 items, simply pick arakune in technical mode and keep the autofire button you set to ABCD and arakune should win almost every match thanks to the ridiculous combo of enhancement + Unlimited, which increases the number of bugs you can summon when the enemy is cursed, at the cost of making the curse shorter, but the enhancement will permanently curse the enemy, so the autofire will keep summoning ridiculous amounts of bugs that will fill the screen, plus due to their number you should advance in depth pretty quickly, making this combo the perfect option to farm items in the Abyss

Section under construction, if you want to help, send me a screenshot of you at a certain floor getting an item drop that is an equippable ability

Items that range from Attack +1 to +9 and the like won't be added since those are easy enough to get, you must be incredibly unlucky to not have one of those
Same with the depth items, those are a bit more rare but still common enough that you should get them all before getting all the equipable skills, those +Depth items come in the following numbers of intensity:
You can use multiple of them before entering the Abyss to start at a deeper floor, using +10+30+50 will land you on floor 90 so you can complete a 100 floors basic Abyss and quickly unlock the enhancement of the character you used in that run
Using from +10 up to +290 will land you on floor 840, close enough to the end, but using them all will land you on the "Infinite" abyss stages beyond 999
And if you used enough depth items that you would end in a floor deeper than the deepest one available in your selected run type, you will just end 10 floors above the limit.
Lastly, there are a few items that don't matter for the Abyssmal Shopaholic achievement, those are the +x All stats, and the Minus Depth items, those won't have a "NEW" highlight when you first meet them, and they won't be unlocked for the shop either.
Those minus depth items can be hepful for farming certain stages like 666 Boss Ragna.

About to lose? Don't worry, I got you covered, every 20 floors we can do a quicksave of our progress, makes sense since a 999 run can take a decent amount of time when you are not using Arakune, this quick save is also deleted after you lose, but not when you load, notice the exploit there?
Key word, AFTER you lose.
While you are losing, simply pause the game, and Alt F4 your way out, and you can go back to the Abyss mode menu and continue from your quick save, as long as the lose animation doesn't fully play, the quicksave won't be deleted, so you have pretty much infinite tries for floor 666 and 999, so you can show the super bosses who rules

Some items are much better than others, these suggestions come from pure personal taste so your mileage may vary.
Victory Rush are really useful, they restore a bonus portion of HP after every victory, so this gives you much needed sustain in the Abyss, regen can help too but if the match end quickly it won't heal much, and for it to be effective, time must be ticking, meaning the enemy can still move, thus, can still hit you, negating the benefit of having regeneration in the first place.

Rapid Cancellers are okay if you are into advanced combos that can use them, or if you have good react time to Rapid and fix that whiff you just did that will probably end up getting punished by the AI's instant distortion drives.

Berserker is nice for damage but also poisons you, high risk, high reward, best used with other sustain skills like regeneration/Victory Rush, or even more damage boosting options like Avenger and Critical Striker/Armor Penetration/Become Unlimited if you want to go full ham and deal obscene amounts of damage.

AI Reductions are amazing, they make enemies much more manageable, and they still work at the later floors, highly recommended skill.

Destruction Drive nullifies the enemy's Distortions which can be crucial at later stages where the AI can punish your whiffs with a distortion with frame perfect inputs, specially when the AI has unlimited characters, very powerful skill.

Become Unlimited is good for obvious reasons.

Character Enhancements are a mixed bag, some are amazing like Arakune's or Tager's, others are just not good enough, or maybe they have some potential if you really know your character well, like Jin's no limit to freezings, that sounds like you could make an infinite combo out of it... try them and have fun with the stuff you can do, infinite flight with Rachel, always use D version specials with Tsubaki, spam chains with Hazama, etc.

If you need some quick money for buying shop stuff, do an arcade run with Stylish Tager in normal, and you will get a nice bonus in money for the completion, but once you have Become Unlimited, simply pick Arakune with his enhancement and Destruction Drive if you have it, then you can easily farm money in the Abyss itself with little effort.

This took a few hours to write and its not even complete, I need to reorder the stuff in the drops section and add some more info, it will be a decent guide someday, I swear...
Finally almost complete, now I just need to get info about the drops I don't have the floor where they might drop
If you have any questions, drop a comment.
Also Lambda 11 best girl