I was fiddling around with the Dungeon Master Mode last night and tried to see how many CR 1/8 Bandits it would take to crash the game. Turns out, it's about 75 on my rig.
It also turns out that using the Dungeon Master Mode Playtest's modes' "Kill Battle Enemies" cheat menu button kills all of the enemies with psychic damage, thus unlocking the "A Battle of Wits" achievement.
Being the industrious fellow I am, I thought "I wonder if the other damage-based achievements can be unlocked this way".
I spawned some scrolls of lightning bolt in a chest and had 20 bandits conveniently positioned in a straight line. I then turned on the "Idle Monsters" and used the scroll. Boom, free "Stormbringer" achievement.
I positioned 20 bandits in a nice square and did the same with some scrolls of cloudkill. Boom, free "Ah, sweet toxicity!" achievement.
While I can't confirm if it's the same for the others (god help the person who spawns 40 magic missile scrolls to get the "That's a lot of magic missiles!" achievement), I'm gonna go on a limb and guess that you can use this exploit and get them as well.
The issue seems to be that the "Playtest" is counted as a normal campaign, and thus are able to get the achievements with next to no effort.