According to Brayarg in the comments, Dead Mage pushed a patch out February 24th that fixes this achievement. Brayarg confirms doing the original requirements and the achievement popped first try.
If you CANNOT
pop the achievement using the actual requirements, let me know in the comments. Until then, I am assuming this achievement is fixed.
Here is the guide:
LaughinGuo in the comments suggested the boss of 2-3: Ruins of Old Barahut
I went ahead and did a quick run. This is definitely the easiest.
The boss reminds me of Whispy Woods from Kirby. This treant has one action where it plants itself into the floor and sprouts curved roots. To damage the boss one must kill the roots while dodging easily telegraphed wood spikes coming up from the earth.
All characters are possible I simply tried Linda since I originally got this achievement with Linda. Would definitely recommend a fully leveled Kevin or Lucy. They have massive DPS outputs.
My first solution: