Rise of the Slime (EP)

This achievement is highly luck-based. It requires two cards in particular: Execution! and Opener.

'Execution!' is the most difficult card in the game to find, it seems. In my many runs, I only stumbled across it twice. Needless to say, if you encounter it, you will want to pick it up. I have only seen it appear in card packs that are awarded following a boss fight (denoted by the skull on the map).

The other card, Opener, is very useful and is a bit more common. Likewise, you will want to pick this up whenever you see it. In fact, the more Openers, the better.

In addition to these two cards, you will want to set up your deck with as few cards as possible and supply it with cheap attack cards (leveled up Lightning cards are ideal). Fewer cards is important because it will increase the probability of getting all the cards you need in your hand at any given time.

I would also recommend picking up any cards that restore mana, allow you to draw cards, or pick cards from your deck/discard pile. Obviously, you should increase your mana stat to the max when you encounter the altar.

To successfully perform the attack, you will need at least one 'Execution!' and one 'Opener' in your hand. You will also need to be next to an enemy for it to work.

'Opener', when used on an enemy will put the 'Mark' status on the enemy. From here, any card that targets that enemy will increase the Mark number. Any cards that don't target the enemy will cause the Mark status to disappear. This means that you must plan your moves out ahead of time (i.e Use your movement first to get next to an enemy or use all of your Draw or Mana spells first).

If you have two Opener spells, it works even better (or one Opener and a Duplicate card). The first Opener will give the enemy one Mark. The second Opener will increase the Mark to 3 (one for targeting the enemy, and another for the card's Mark ability). The Opener cards will also spawn a Punch attack each. These 2 punch attacks (1 Mana each) will get the Mark counter to 5. Now, you can use the Execution attack (0 mana) to kill the enemy and unlock the achievement).

If you only have one Opener card, you may want to use a leveled up Lightning card (since it only costs 1 mana and will spawn a lower leveled Lightning card after use).

This may be a bit tough to understand for beginners. However, once you have put several hours into the game, it will make a lot more sense.

Highly luck-based though.

EDIT: Here is the clip of me getting the achievement. I already set everything up before I started recording; moving next to the enemy and using mana spells (which is why I have so much Mana). The target icon above the enemy is the 'Mark' counter, which you need to get to 5. Just remember, no wasted moves or else it will reset the counter.