Plants vs. Zombies: Battle for Neighborville

Easy breezy. While playing with the Acorn, transform yourself into an Oak and start roaming around the mission areas. As soon as you see two or more regular zombies (coneheads, brown coats or TV heads), walk around them until they are side by side and press RB. The Oak will throw a rolling tree at them for instant kill. The achievement will pop when you kill 20 groups of at least 2 zombies.

Beware:
(i) zombies holding doors and flags protect the others from the tree branches;
(ii) if the branch hits a wall or object on the street, before reaching the zombies, it will be destroyed and no one will be killed by it;
(iii) to kill heroes, shoot them first with your regular gun and use the "roll for damage" as your final blow.

If you’re struggling with doing this in regular play, you can try this:
Go to the Impninja/Trueninja bounty in the northern part of the mount steep map for plants. Kill the enenmies in the area around the bounty and switch to the oak. Now when you start this bounty there will be 3 imps, 1 of them is the boss that has regular health and two others that are killed by a single hit. You have to hit both of the small imps with your RB ability simultaneously shortly after they spawn in order to get progress for the achievement. If the time between hitting them is too long you will not get progress because you will fail the bounty and the other imps will disappear before you get the kill. You can restart the bounty every time you get a double kill (or not), so you have a chance every 8 seconds or so.

It’s a little random due to the imps spawning differently and the randomness of the oak roll but it took me about 15 minutes by myself to get all 20 double kills required for the achievement. It works easier if you’re with a friend who can always restart the bounty for you while you can wait at the back of the wall ready to press RB (or another spot that works for you) If you find a location that works for progress, just stay there, don’t move your reticle and have him start over and over and you will be able to get multi kills on basically every try. Good luck !

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.