Slay The Spire

A fairly reliable way to do this is with a Searing Blow+ deck against the Awakened One alongside some method of playing it twice. Searing Blow can be upgraded any number of times so you'll need to focus on routes with Rests(to upgrade), Merchants(to remove cards) and Unknowns(?)(for the chance at upgrades and card removals).

The only requirements for this method are to have:
Searing Blow+ (The higher the better, ideally 9+)
Vulnerability (Fear Potion / Bash / Thunder Clap)
A means to play Searing Blow twice:
- Duplication potion (Play next card twice) or
- Necronomicon relic (First card costing 2 energy each turn is played twice) or
- Double Tap (Play next attack twice, requires it to also be drawn on the first turn though)

Things which can help the run/boss:
Extra energy (Seeing Red, Energy potion, Velvet Choker, Ancient Tea Set)
Attack potion (an extra free attack for the boss)
Paper phrog (Enemies take 75% more damage when vulnerable)
Gamblers Brew/Swift Potion (Increased chance to draw the card on the first turn)

Here's a clip of me finishing the boss's first phase and screenshot of the run. With the Awakened One, finishing its first form is enough - then I lost to the second form:

INFINITE LOOP OF ATTACK

Method with the Watcher (the mantra lady, 4th and last character to be unlocked).

I'd like to point out that I don't watch Youtube nor other player’s channels nor have I looked up any guides on the net, so I'm extra proud to have come up with a method of my own, which also helped me to unlock at least 2 other tough achievements in this game using what I'm gonna describe right now.

You only need 3 cards (and two of them you already start with since the beginning of the game) and a relic called Violet Lotus which gives you an additional energy point whenever you exit Calm.

The cards you are gonna need are:

- Eruption ("Deal 9 damage. Enter Wrath")
- Vigilance ("Enter Calm. Gain 8 Block")
- Rushdown ("Whenever you enter Wrath, draw 2 cards")

From the above, Rushdown is the one you are going to have to acquire along the way.

Using the three of them in the right order will make you enter an infinite loop of attacks until any enemy is dead. It works not only on bosses but it might help you defeat any foe (the Transient or any other high HP enemy as well). The downside per se is that the damage done with each attack is small, so it will take many hits to end the fight, but it works great and that's what matters in the end. Here's the step by step:

1- First, play Rushdown to apply its effects on this turn/fight. 2- Then play Vigilance to enter Calm and 3- finally use Eruption to enter Wrath and attack. That's it. Rinse and repeat steps 2 and 3 to keep attacking.

Having those cards upgraded is a plus (or even a necessity in case you only have 3 total energy to spend, since Vigilance costs 2 and upgraded Eruption costs 1). When you exit Calm you earn 3 more energy, which is essencial for the loop since you will keep spending those earned energy points on Vigilance and Eruption, respectively, over and over.

You are also gonna need to have a hand of about 7 cards in total, because when the Rushdown effect happens you need it to bring back those 2 other cards that you are using to keep the loop and attack, that is, Vigilance and Eruption. If you have more than 7 cards total you risk breaking the loop in case one of those 2 doesn't return to your hand when redrawing.

In order to achieve only 7 cards or less in your hand, simply take advantage from the beginning of the game to get rid of the other ones you don't need. There are plenty of opportunities between some of the ? (question mark) events and also the removal services found in shops. For me it seems almost like the game notices that I'm playing this way (minimalist deck) from start and throws a lot of such opportunities.

Extra info:

In case you are lucky enough to acquire Rushdown by the time you reach the first boss, great. Me? I unlocked this achievement at the final boss (before the Heart), on act 3, and before that I had used this same technique on the Transient for the other achievement. Because I was learning as I went (used no guides), I had a few cards over the 7 total in my hand but I also had a Blasphemy card ("Retain. Enter Divinity. Die next turn. Exhaust.") and a potion which allowed me to enter either Calm or Wrath as backups. Those extras helped me get the loop started. So I first entered Divinity, then used the Rushdown card, then a potion to enter Wrath. By doing that, I was finally able to have Eruption and Vigilance in my hand to start the proper loop as explained. I was unlucky to get the toughest boss (in my opinion) as the enemy, the Awaked One, but as soon as I killed him the first time the achievement unlocked.

About the Violet Lotus relic:

Although much on this game is random, there are certain patterns you can learn by experience or you could, for instance, read the wiki dedicated to this game. I've noticed thru experience that the Violet Lotus can appear at least from these 3 different events:

- At the beginning of the climb, (IF you have reached a boss on your previous attempt and) if you decide to swap your original relic (Pure Water) for it;

- Before the first boss (although I believe it could also appear after him, since the relic's order as a reward or acquirable item tend to change every time). I'm adding this just for reference, for it has happen to me multiple times and it could help a player trying the achievement for defeating a boss on turn 1.

- As a reward for defeating the 1st boss on chapter 1. This was what happened to me when I beated ascension level 20, although it might be worth to remember that it is NOT always shown as one of the 3 possible rewards, it doesn't happen often or at least every time.

EDIT: Qwanimal suggests, in case you can't find the Violet Lotus relic, players can use the Inner Peace ("If you are in Calm, draw 3 cards, otherwise Enter Calm") card instead of Vigilance since it allows players to enter Calm costing only 1 energy.

I Just got this with the starting condition of First 3 battles enemies have 1 HP. I got two games in a row where a path would only have 2 battles to the boss.

Only issue is if a ? has a battle. Restart and keep trying.

Probably best to simply be aware of this achievement and look for opportunities as you progress. I realised I could have killed the third boss first turn so I pressed the home button and quit the game before finishing my turn. Even on ascension 5 it was easy at that point, but only due to previous luck during the run.
I beat the two donut bosses with the poison card that deals total hp damage to the remaining enemies. This wasn't necessary however because I did 349 excess damage so I could have killed the other donut in a single hit.





I actually used the artifact you get after a failed run which allowed me to kill two elites with just 1hp. That way when I added cards it was tailored to the two artifacts I got, I passed on the initial cards for this reason.

PS I once got all the way to the final boss using this method and killed the slime with one hit. Not sure why I didn't get the achievement then.