Merge & Blade

Heads up: this achievement requires a lot of grinding and will likely be very frustrating for you. I estimate that it took me between 15 and 20 hours from floor 1 to floor 35. Also, this is the first guide I've written and English is not my first language, so please excuse any mistakes in that regard.

I used two different strategies for the Into The Dark levels, both with their own advantages and disadvantages, but generally the same tactics apply.

1. Puppy Character

The Puppy is the strongest character in the game, if you're lucky. Stunning enemies in a 2x3 grid is pretty good on its own, but you will only do 23 damage per enemy, which is less than a level 1 Farmer. However, getting a "Character ATK" card (the one that adds damage, not the one that will multiply damage) will give you a huge damage boost and make you extremely powerful in the early game. When playing with Puppy, getting these cards is the #1 priority.
A big disadvantage of course is that if you don't get these cards, or if you don't get them early enough, it will be a lot more challenging. Another disadvantage is that you will have to place Puppy in the first or second row, and on the lower floors some enemies will be able to kill you with few hits, even on the early rounds. Info, as it's a bit confusing: when I say "lower floors", I mean floors 20-35, as you go downstairs, not the ones with low numbers.

2. Prophet Character

Compared to the Puppy, the Prophet is safer to play and generally relies less on luck. He will do a lot of damage to a random enemy from anywhere on the grid, but is very slow. Early on, I've found that the best tactic with him is to plaster the grid with Farmers, Blacksmiths and Footmen, intentionally not combining them to have as many units on the grid as possible. Then, position the Prophet as far out of the line of fire as possible and use the units as cannonfodder while the Prophet kills all enemies. Here, the "Character ATK" cards are still very good, but reducing the attack time and getting more health is more important. With only about 1000 health, having any of the stronger enemies attack the Prophet is an instant death sentence.

Getting It Done

As of April 15th 2024 there is an exploit which will make this a lot easier: normally, when losing multiple times in a row, you will be sent back to the upper floors, erasing some of your progress. However, Alt-F4-ing before your character dies will prevent this from happening. I wasn't able to try if this also works on Xbox, but closing the game might do the job.
With the exception of the character-specific priorities, you should first take "Academy"-cards to try to unlock the Priests as fast as possible. After that, take cards that will give you more turns and combine your units to actually get Priests on your grid.
Now, get more "Academy"-cards until you unlock the Barbarians. These will be the highest units you will need. If you get the Alchemists on accident or because there was only a +2 or +3 card available, it won't ruin your game, but generally, I've found the Barbarians to be the better choice as your main attack units in the later rounds. The reason for this is that I always tried to fill the entire first row with Barbarians to get the most out of their 1x3 attack pattern, allowing 3 front row units to attack the strongest enemy at the end of a round. Barbarians also attack faster, which can make an important difference. Getting anything higher than Alchemists is overkill, you will most likely never get them on the grid anyways.
In 95% of cases, I have gone for the most difficult stages. Especially on the lower floors, I found that the difference in enemy difficulty between easy, medium and hard dungeons is not too big, but the rewards on the hard levels were almost always better. Having good cards with better rarities is not guaranteed, but you need to take every advantage you can get.

General Tips

• Try to get high combos. Early on, having more units on the grid is better than having a few strong ones, but you should also try to build up swaps so you don't run out of them at the end and have to take a card.

• Speaking of swaps, your main use for them should be to get your character out of the line of fire and to get the stronger units towards the front of the grid. Another good use is to make a combo that will unlock a new unit for you, but I don't recommend on using them just to try and build high combos and make room on the grid.

• Make sure you know the attack patterns of enemies. Some will only attack units on the second row, some will attack on the third row, some will attack diagonally, etc.

• Quantity is often better than quality. Depending on the circumstance, having 3 units on the grid might be better than combining them to 1 stronger unit

• Use up your recons on the later, more challenging rounds. You can also use 1 or 2 in the beginning, but definitely save 1 for the last battle.