This trophy was formerly glitched in a prior version, but as of Update 1.4 it should be working.
You can obtain Crate by completing Proving Grounds/Campaign missions as rewards; Rarely they can drop after killing an enemy in Campagin, this will pop after getting 20 of them.
If you had over twenty crates before the patch(you can check and see be looking if you have the crate avatar unlocked, it's the same image as the trophy) , this will pop after getting another crate.