Jock ( Red tops ) - Level 5 and level 6
Hillbillie ( the guys with the shotguns ) - Level 6
Singing Guy ( red and white top white hats ) - Level 12
Soldier ( the ones that don't wear helmets ) - Level 9
Cop - Level 5 ( not sure if the cop is nessary for the achievement, I've added it just in case.
The levels I have posted are imo the easiest to achieve. There all at the start of the levels and only require one possession for the achievement tracker too register. All in all I think its just four you need for the achievement Jock, Hillbillie, singing guy and the solider. I'm not 100% on the cop.
Achievement unlocked for me after getting each of these. Very simple achievement.