Act 3-2 (Evolution) on Casual difficulty is a great place to earn this achievement with no effort at all, play up to the part where the stranded guy falls to his death on the wooden planks and gets eaten alive by lambent wretches.
Purposely fall down to where the stranded just fell and equip your grenades, gather three wretches together, press
while next to the wretches to tag them and your frag will detonate taking at least 4 wretches with it every-time.
Then revert to last checkpoint and do the same over and over till your achievement unlocks.