This achievement can be unlocked by vanquishing 2 or more zombies 20 times using the
ability for Acorn's second form, Oak.
This achievement isn't really that tricky, it's just a bit time-consuming.
The easiest way to unlock this achievement is by playing lots of Ops matches (preferably private matches, so no other players can interfere) and using the ability near groups of unprotected Browncoats, since they perish in one hit. It is really tricky to pull this off with other zombies as you will have to lower their health down first.
Get 20 multi-vanquishes and the achievement is yours.