I love them. You love them. Everyone loves them.
Bring yer ideas here, lads!
Here are my own ideas (some of them might be hard to achieve, I know (you can't achieve none of these achievements by attacking or destroying your own units)):
Achievements related to Gorillas:
Kill 20,000 (or X) enemy units with Gorilla's "Ground Punch".
The supremacy of a gardener
Kill 30,000 (or X) enemy units with Gorillas.
Make your Gorilla survive direct hits with nukes, ion cannons and vortex singularities in a single life (without healing its HP).
Unlocks: Special skin for Gorillas!
The bigger they are, the harder they fall
Kill 12,500 (or X) enemy Gorillas.
Destroy 10 (or X>6) enemy units with a single smash.
A bit heavy
Squash X enemy units with your feet (walk over them).
Unlocks: Spiky boots reskin!
Kill 5 (or X) enemy units in a single charge. (Note: This is my idea!)
Achievements related to Reapers:
Reaper of souls
Kill 30,000 (or X) enemy units with Reapers.
They really met their match!
Kill 20,000 (or X) enemy Reapers (Note: This achievement's name makes more sense in Polish language).
Long-range array of doom
Destroy 15,000 (or X) enemy turrets by shooting outside their firing range.
No room for purifiers!
Destroy 5,000 (or X) Paladins using Reapers (Note: "Paladin" is my idea too!).
Unlocks: A skin for Reapers.
Achievements related to stats:
Kill 5,000 enemy units in a single match.
Unlocks: Weapon Mechanics opening screen.
Hail to the king, baby!
Win a total of 500 (or X) times.
Unlocks: A few reskins for kinetic weapons + nuke (projectile) reskin (Yes, it is kinetic, too, I know).
Achievements related to WMDs (Weapons of Mass Destruction):
Kill 50,000 (or X) enemy units with nukes.
You never know when this day comes
Kill 50,000 (or X) enemy units with ion cannons.
Still no radiation fallout
Launch a total of 5,000 nukes.
Unlocks: Nuke (silo) reskin.
Fire a total of 5,000 ion cannons.
Unlocks: Ion blast recoloring.
Once upon a time
Launch 50 nukes in one match.
Can't touch this... literally
Fire 50 ion cannons in one match.