I like to play support classes on FPS (I play both Eng. and Medic) but considering the general demographic of these games I think my earlier suggestions would be the most effective in terms of getting a good K/D ratio for the least amount of pts.
Playing Killzone for KDR is probably the worst idea possible. You WILL be dead last in the leaderboards and leveling up will take FOREVER. And your teammates will hate you for it.
Seriously, its an objective based game, play for objectives. In high level play nobody complains about marksmen in warzone because they don't show up that often, since they're bad at completing objectives. And when you do get the occasional d-bag that just hangs back and snipes the entire match going for his KDR, the team he's sniping is usually laughing their ass off because it means that his team is essentially down a player, and his team is pissed, because they are effectively down a player. The respawn time is so short that kills have to MEAN something.
And lets put this numerically:
Kill = 100 points
Headshot = 150 points
Planting an Explosive = 350 points
Explosive blows up = 650 points
Getting both explosives planted and blown up = 2500 points
Downing the assassination target = 650 points
bleeding out the assassination target = 350 points
Delivering a speaker = 1000 points
Defusing an explosive = 350 points
Surviving an assassination = 1000 points
Capturing an area = 300 points
Capturing a TSA = 150 points
Reviving a player = 50 points
Repairing something = 50 points
Basically GG have made it so 1 objective is worth 10 kills. Playing for KDR gets you damn near nothing, and its a waste of time. Eventually you'll be good enough to get a good KDR and complete objectives, but it takes time.