Sharp shooter, Gun Game or Sticks and Stones, constant UAV mixed with the worst players there is = lots of kills.
Try capture the flag. Stay at your flag and play defense this is a good way to contribute. Run uav, counter uav to help your teammates who are capping.
Defending in CTF is not an easy job for someone not that great at the game, you need a very high level of situational awareness.
IMO if you are bad at the game there might not be a perfect game type. In my experience of ctf defending the flag is a lot easier than running it. If he's not that good sitting back and watching the flag might be a good way to help support his team.
Play some Custom Games against the Bots. Sounds silly but it really does help you develop your skills.
I got a 57 killstreak against bots today, first time playing them, 4 of them and my brothers 5 year old son, was quite a good laugh but would definitely advise against playing them trying to improve at the game, only thing I think you could gain is map knowledge.
Its not for anything BUT map knowledge and weapon handling.
It really helped my game considerably. You learn sightlines and such pretty well. Its nice being a no pressure environment where you can experiment with setups and try new things without dragging a team down or getting beat up on.
True, maybe worded that wrong saying better at the game, meant better at multiplayer against real people and yes, it defo would help at getting used to weapons as well.
There's combat training too it's a mixture of bots and players and you gain xp. None of your stats count though.
My daughter plays that now and again and EVERY time she plays there is one guy who gets like 100 kills with VSAT loadster and dogs, not very much help if your trying to improve at the game, pubs are the best for improving, skill based matchmaking makes sure of that or find some friends and play as a group, 6 bad players playing together are better than 6 decent randoms.