victim of circumstance - have the most kills in the game... And loose (slayer modes)
flag/bomb/ball spree - get 5/10/15 etc kills with an objective item.
bait - kill an enemy who is returning a flag/ disarming a bomb
the best defense... - assassinate an enemy who has over 50% of an over shield remaining.
catch - kill an enemy with debris from an explosion.
back seat driver - swap into a gunner seat from driving, get a kill, and swap back in 5 seconds.
phalanx - get a defender kill with a jackal shield equipped. (would require jackal shields to be a thing. Preferably dual wielding in general and jackal shields being 'unique' so you can only carry one but in either hand)
efficiency - kill more than 1 enemy with a single sniper shot.