There's a few major hurdles we're going to have to overcome to complete this achievement:

• Ability to defeat Death
  
• Prevent Death from killing the enemies
  
• Avoid dying to the enemies
  
• Avoid dying to Death's Dementor ability
  
• Prevent ourself from dying to Death's attacks
  
• Avoid accidentally killing the other enemies ourself

Let's briefly discuss how we're going to accomplish these. The specifics will be covered below in detail.

Ability to Defeat Death
Level 8 Death heals whenever another enemy dies. Additionally, he becomes immune to damage every 111 damage he takes, and he can't take more than 111 damage from a single hit (not entirely true - we'll get to that). Additionally, if he gets dispelled, he heals for 111 damage - so we can't keep dispelling the invulnerability. As such, we can't expect to kill him in a single turn, either with a single very high damage attack, or with an infinite chain of damage cards. We're going to do it slowly, over many, many turns, and we're going to bypass his damage limits to an extent with critical hits.

Prevent Death from killing the enemies
There aren't any enemies on floor 1 that are immune to Death's attacks, so we only have one real option - but lucky for us, it's the easiest part of the strategy.

The demon boss (the icon looks like a head with 2 curly horns) has an ability wherein when he dies, he respawns as a 50-hit-point stone version of himself, and one turn later, he respawns again as his 18-hit-point self again. If Death kills his 'living' form, he turns to stone, then immediately turns back into his 'living' form again at the end of our turn. As such, we can let Death kill him every turn, and he'll continually regenerate - as long as we never kill either form.

Avoid dying to the enemies
This goes hand in hand with the above step. His stone form can't attack - and since he'll be spending every one of his turns turning from his stone form into his living form, he'll never be able to attack us, after the first few rounds, anyway.

Avoid dying to Death's Dementor ability
This is the first real difficult hurdle to overcome. On Death's first attack at level 8, he applies an unremovable debuff that kills us 6 turns later. Lucky for us, it's possible to dodge it. We'll use Wind Feather equipment cards to facilitate this.


Prevent ourselves from dying to Death's attacks
Since Death gains 1 attack each time he takes damage, he'll eventually be doing enough damage to kill us in 1 hit. As such, we'll need to avoid that. There's a few options here, but the easiest is to equip a Heavy Armor, limiting the damage we take to 6 per hit, which we can trivially overcome with armor.


Avoid accidentally killing the other enemies ourselves
Since we need the fight to last a while, we need to avoid killing the boss before Death dies. As such, we need to avoid any cards that deal damage to all enemies, or a random enemy. This includes many normally very powerful cards. Be mindful of every card you pick.

Since a specific boss is required to facilitate the strategy, we'll need to restart the run immediately if it doesn't have that boss at the end of the first floor. (Open the Menu, choose 'Quit', then choose New Run from the main menu.)

Any floor 1 with the demon boss is viable, but there's a few other features that can make it better. Some things to look out for:

4+ Elite enemies - Since we need a very specific set of cards to make this work, we want as many card picks as possible before entering the fight. Elite enemies give 2 card rewards instead of 1, so we want as many as possible.

9-10 Combat Encounters - Again, we want as many card picks as possible. Yes, we can get very lucky with Events, but it's a lot safer to take combat encounters and guarantee the card picks. With 6 normal and 4 elite encounters plus a shop, it's relatively easy to get the cards we need.

A few things are required to make this work:

• Dodge Chance
  
• Repeatable, Reliable Healing
  
• Heavy Armor (or a way to get it), or a reliable way to get lots of armor each turn
  
• A way to get lots of attack, or a high critical chance, or both

Dodge Chance
The easiest option is Wind Feather. It's a basic card, so you get 3 of them, and with all 3 in play, you have a 45% base chance to dodge attacks.

This is pretty good. Another, considerably worse option is Shield Down. This provides a 50% dodge chance, but you need to use it the turn before Death shows up, and it removes all of your block, which may be dangerous until you're properly set up.

It may also be possible to avoid dying to the Dementor effect with Love; getting this on floor 1 is incredibly unreliable, and I haven't had a chance to test this. If you happen to get it, you may as well give it a shot.


Repeatable, Reliable Healing
The basic Heal cards won't cut it - we need to guarantee that we're able to heal consistently over long periods, and it's possible to end up with all of the Heal cards in the discard pile without careful management. As such, we want a better option. The best choice is Holy Guardian. It's common, so you get 3 copies, it's easy to find, and best of all, it's passive healing, every turn.

Another worthwhile option is Flash Heal. It's arguably more reliable, since you can't lose it into the discard pile, but having to play it every turn, sometimes many times, will get annoying.


Armor
Heavy Armor solves this problem completely.

Failing that, Charged Shield will generally work.

In a pinch, Staff of Order can potentially be enough, if you get lucky with your other cards.

There are plenty of other viable options here, including cards that prevent all damage for a turn or from a single attack, or cards that grant considerable amounts of armor as a one-time effect, but the above options are the ones we want to try for.

Attack and Critical Chance
For Critical Chance, Punch Gloves is the only real option. Critical hits can bypass the 111 damage per hit limit, potentially making the fight much faster. This isn't a hard requirement, so if you don't get the gloves, don't worry about it too much.

The best option for Attack is Hero's Sword. It's automatic and a one-time investment for potentially unlimited attack.

A functional alternative is its little brother, Blade Fury.

Don't bother with single +1 attack equipment; they're too minor to be worth it, and we're already going to be running a relatively thin deck.

Honorable Mentions
There's some cards which, while not required, will make everything considerably easier for us.
Necromancer's Robe allows for infinitely long turns when used in conjunction with repeatable card draw such as Ring of Infinity. To go truly infinite, you need 3 of them and no cards that cost more than 1, but even just having two in your deck (one card reward pickup) will considerably extend your turns.


Since Death puts a curse in our hand every time he attacks, we'll likely end up with a number of curses in our deck. Having a Voodoo Doll lets us turn those into damage, which is very helpful when trying to extend our turns as far as possible.


Cards that let us copy cards in our deck, such as Illusory Equipment and Buried Secrets, can let us get more mileage out of our equipment cards. With a few of these effects, a single Wind Feather reward can be duplicated to gain a 100% dodge chance, and a single Necromancer's Robe pick can be duplicated to get us infinite energy in any deck, among other things.


Pick 1 cards like Waste Picker and Fate can help fill in the gaps if we didn't get all the cards we wanted. Waste Picker especially can, over a long enough time period, get us any and all of the equipment we failed to get from card rewards. It's almost always worth picking up.


Heavy Shield Bash is my preferred finisher, and with Charged Shield can completely cover both your damage and armor needs in a single card.

Easily as important as picking the cards we need, is avoiding the cards we don't want. There are a lot of these. Do not, for any reason, take one of these cards either as a card reward or off of a Pick 1 card. If you can't avoid it, do not for any reason play the card.

Damage To All Enemies cards
Any card that deals damage to all enemies - even just 1 damage - can't be used. By the end of the fight, you'll likely have enough Attack to turn a 1 damage attack into 18 or more, and that will let Death kill the boss's Stone form, and end the fight. Here's some examples to watch out for, but any Damage To All Enemies card will screw you.



Forbidding Hairband
This should be obvious, but we'll need to recycle our cards repeatedly. Destroying them is bad. This also goes for any other cards that indiscriminately destroy our cards. Cards that destroy themselves when played are generally fine.


Damage To Random Enemies
For the same reason we can't play damage to all enemies cards, we can't play damage to random enemies - we'll accidentally kill the boss before Death. Again, these are only examples, as there are too many to list.


Seconds Counter
We'll be taking some long turns, and 15 seconds just isn't going to cut it. This isn't technically a run ender, but it'll make things unnecessarily difficult. Avoid it.


Cards that Destroy Equipment
Our equipment is what's enabling this strategy, so we obviously don't want to destroy it.


Reckless Action
Not technically a run ender all the time, but Death becomes invulnerable every 111 damage he takes. This will mill your entire deck. This is generally a bad thing, especially if you're playing with a 1-card hand because of Ring of Infinity. Don't do it.


Enemy Dispel Effects
Since Death heals for 111 each time he has an effect dispelled, this will fully heal him each time you use it. This is generally bad.


Flash Storm
Death gains 2 block each time he takes damage, so this card deals 10 damage, he gains 2 block, this card deals 10 damage again, he gains 2 block, repeat ad nauseam. This sounds great, except that he's capped at 111 damage before becoming invulnerable, but he still gains the block when hit, so this card keeps trying to do damage. If you play this against Death, the game will crash.


Love Call
I hope I don't need to tell you why this is bad.

The fight itself is pretty straight forward once you have the deck constructed. Here's the key points:

Don't damage the boss. If you have to deal some damage initially to gain block or clear your hand, that's fine, but once Death shows up, direct your damage at Death to get his per-hit damage up to 18, then let him do his thing.

Prioritize Dodge cards before Death arrives If you don't dodge the Dementor effect, you can't win. Sometimes you won't get the dodge cards in time - that's okay. In a pinch, you can Alt+F4 the fight, and when you continue the run, you'll restart the fight. Technically an exploit. This may get fixed at some point. Up to you whether you want to take advantage of it in the meantime.

Don't try to win too quickly. Don't deal too much damage to Death too quickly. Every time you hit him, he gains 1 attack, and he's immune to detrimental effects (like 'Enemies can't attack' or 'attack down' cards), so you'll need to be able to take increasingly large hits. Until you get your equipment in place, he can still kill you. Once you have Heavy Armor or a few Charged Shields, you can start going on the offensive.

Death heals for 22 every time the boss changes form. This will happen twice each turn - once when Death kills him, and once more on your turn when he changes back, so you'll need to be able to overcome 44 points of damage per turn. This is why you have increasing attack cards like Hero's Sword in your deck. You'll need to wait until you have enough attack that you can take him down past the invulnerability thresholds far enough that he won't get healed up past them again.

To clarify this, Death has 666 hit points. Every 111 damage, he becomes invulnerable for a turn, so that's at 555, 444, etc. If you damage him down to, say, 530, he becomes invulnerable, then the boss dies twice, he'll heal up to 574, and when he drops to 555 again, he'll become invulnerable. As such, you need to be able to deal more than 44 damage past the threshold to him. As your attack gets higher, it'll become increasingly easy to do this, until eventually you only need to use 1 attack card per turn. If your attack crits, you can deal up to 222 damage in a single hit, making the fight progress much faster.

Avoid cards that repeat a lot of times. Cards like Double Kick sound great, but the reality is that the game doesn't perform well with fights that last this long or include this many factors. If you Double Kick with too many effects in play that trigger off of damage (e.g. Charged Shield) and too many cards played previously in the turn, the game will hang, and you'll be forced to restart the fight.



It may take a few tries, but this strategy will earn you the achievement... at least until the cards or boss mechanics are changed. Get it while it's hot!