Gloomhaven

I did this with a Level 6 Doomstalker on Scenario 3, Inox Encampment.

You could probably get this normally with the Doomstalker, but I found this to be the easiest way.

I brought 4 Mercenaries, however I only used the Doomstalker to attack. I also brought a Level 5 Quartermaster and used his combos to allow the Doomstalker to use a Major Stamina Potion every turn, ensuring he would never exhaust. The other Two Mercenaries were tanks, and just stood in front of everyone while the Doomstalker shot everyone.
I ended up finishing the Scenario with 207 Damage.

Quarter Master Combo - Scroll of Recall, Reforge, Continual Supply, and a Major Stamina Potion. You set up Continual Supply, then play Scroll and Reforge every turn getting back your Potion and the Card you discarded to Continual Supply.
Then on the Doomspeaker, just use your choice of two cards, use a Major Potion to get them back, and then use Continual Supply to get the potion back.

Once you get your Twentieth Kill the Scenario will end at the end of the round, and you should unlock the ''Master Assassin'' Achievement!