Oddworld: Soulstorm

Types of Sligs are based on the weapons they wield, so there are 12 different types, of which only 11 can be possessed. The melee Sligs with tinfoil hats cannot be possessed. Those that you can possess are spread throughout the game, with the exception of one which will be detailed further down. The Sligs you need to possess are as follows:

Infantry Sligs
Assault Rifle
Shotgun
Rocket Launcher
Flamethrower
Gatling Gun
Sniper - This can only be earned in Level 5: Sorrow Valley. During the section where you used a possessed flying Slig to clear out some other Sligs. You'll need to transfer your possession orb from the flying Slig to the sniper Slig in the final section. It's hard to explain, so click here for a video.

Flying Sligs
Assault Rifle
Shotgun
Rocket Launcher
Flamethrower
Gatling Gun

The earliest you can unlock this trophy is in Level 7: The Hijack, which is where you see Gatling Gun Sligs for the first time. Level Select can be used to mop up this trophy if necessary.