PRACTICALLY UNMISSABLE (unless you hate shooting aliens, in which case, why are you even here)
MISSION: Mission 1: A Smell of Petroleum Prevails
DIFFICULTY: Any
CO-OP: Unlocks, individually
METHOD: Shortly into the first mission, kill a Aludran Microhopper (one of the green, hoppy, frog/dogs). The place is lousy with them. They can also be found throughout the campaign.