Reverie Knights Tactics

I've killed 4 enemies with one skill before a couple of times but I didn't figure out why I wasn't getting the achievement until Act 3 so that's what I'll detail below.

In Act 3 when you get to the city Rarnaak there's an Armory and a Bridge battles. After getting the Spiked Gauntlet from the Armory give it to the character with the highest defense and go to the Bridge. Have your character with the gauntlet go to the tile to the left of the bear trap. In my case it was Fren and I put all of his skill points into defense whenever he leveled up and he had the Amulet of Shelter equipped so the thorns from the gauntlet did 12 damage back to whoever attacked him. His defense was obviously high so I just had him stay on that tile using Greater Guard and all the other enemies moved up to attack him and got everyone's health down to below 10 or to 1 in most cases. The spike damage won't kill anyone for some reason so just wait till everyone is below 10 or so.
Here's the part that explains why I couldn't get it earlier... the Dogblin enemies apparently don't count as "enemies" for this achievement so go ahead and kill them when they are weak so the other 6 "enemies" can bunch up closer to Fren. When they are use whoever in your party that can do an area of effect attack that can hit at least 4 of the 6 and - boom achievement. Focus attacks are good for covering large areas and in particular "full" focus attacks able to use all 100 points are your best bet so keep that in mind beforehand, try to max out someone's focus at the Armory first.