Acrid can be found in the top-right area of the Sunken Tomb (or rarely the Ancient Valley) level in a cage. After activating the crate, it will open, releasing an NPC, hostile Acrid. He has a good amount of health, but is easy to kill with drones. Once defeated, the playable character Acrid will be unlocked.
Acrid will only spawn in one of the three layouts of the area.
Note: If Acrid is proving tough to beat he can be lured and made to fall off the map. This will still unlock the character.
Note: This fight does not despawn any on map enemies and they will continue to spawn during the fight. A way to avoid this is to beat the teleporter and then return to the crate.
Note: The cage only spawns on one variant of the map. If the crate does not spawn, then that means that the Artifact has spawned instead.