Armello

All credits to Dolop O'Dog for the strat from the SteamCommunity. I'll copy paste it below with just a few changes which can help to adapt your game into getting this achievement.
https://steamcommunity.com/app/290340/discussions/0/17433461...

Preparation:
Start a standard single-player match.
Select River, and equip yourself with Celestite and Spirit+1.
Select Zosha, Mercurio, and Thane as your opponents. Equip them all with Turquoise and Spoil. (Note: Mercurio can be substituted, but is not ideal)

Quests:
During the match, focus on Wit and Spirit quests. Likewise, take any treasure or follower that boosts Wit or Spirit. The most important followers to watch out for are Apprentice and Warlock. The most important treasures are Helm of Heroes, Masquerade Mask, and Wyldfyre Staff. None of these cards are necessary, but they are helpful.
As for opponents, the more they are, the better. Always try to summon banes, make guards get out of the castle, etc. Remember they all count for the achievement.

Cards:
Draw EXCLUSIVELY from the Spell deck. Depending on the result, do the following:

Teleport: Save this card : MOST IMPORTANT.
Evil Eye, Focus: Cast these to boost Wit or Spirit respectively. Focus should be cast before Dusk.
Haste: Cast this when there is a grouping of two or more characters on the board with 1 HP
Any Damage Spell: Cast these to reduce any nearby NPCs to 1 HP, or keep them in your deck for later on. Bear in mind, all opponents will lose 1HP every dawn.
Banish, Blizzard, Malice Rising, Tanglevine: Cast these to group characters, and control or limit movement.
Any Other Card: Burn or cast ASAP.

Strategy:
Play the quest and card strategies until you draw Teleport. Try to always maintain enough magic to cast Teleport.

You're now on the hunt, keep an eye out for any grouping of three characters on the board. There are a few outcomes when doing this:

All three characters have 1HP: Teleport near them.
Two characters have 1HP: Teleport adjacent to a 1HP character and a healthy character. Use any Spirit cards to damage the healthy character. If you reduce them to 1HP, you should be able to use your remaining 3AP to eliminate all three characters. If you don't reduce them to 1HP, exit the game and continue from your last checkpoint. You can repeat as many times as necessary to get the correct outcome. If there is no viable outcome, reset from your last checkpoint and continue hunting.
One character has 1HP: Ignore this group and continue hunting.

You'll likely get opportunities in the last turns of the game.

Be patient. It took me 6-7 games using this strat, with loads of frustrating moments. Probably the most luck-based achievement I've ever gotten.
Good luck !

Just got this. You're going to have to get pretty lucky.

You essentially need to have three enemies surrounding you on one health each at the start of your turn. You then attack each one and kill them with the bow attack that River does.

There are other cards that can loosen the requirements up a bit. In my case I had two enemies next to me with one health each and another not too far away. I killed the first two and then used a teleport card to reach the last one.

You can also weaken enemies with spells etc. Beyond that, there isn't much you can do to increase your chances. Just play as River a ton and eventually the opportunity will come up. I was actually going for another achievement at the time, but abandoned it as soon as I realised I could get this one.

Good luck.