This achievement is called "Oh, no you don't"
You get this achievement for killing a Ram using the tackle skill.
The rams are the giant no armed charging guys. It is best to get there health as low as possible before trying to kill them with a tackle as the tackle skill does very little damage and the achievement can still be unlocked as long as it is a tackle that kills the ram.
(*Note: you must have at least 2 Ranks in the tackle skill in order to do any damage with tackle. You should see damage appear above the Rams head when you hit him. (credit goes to mattcrossUNC))
(*Note: Sam B is the only character that gets the tackle skill. (credit goes to Mercenary Girl))
(Note: Full Credit Goes to Cro11 who wrote this for the Xbox 360)