Autobombs do wonders for ya. Can't tell you how many surprise splats I got...or received from an autobomb that they/I didn't see coming.
But honestly...it depends. I got to S+ easily on TC, skipping all of B and A to S, which made it easier. Every other mode has been a nightmare...specially when you get people who just go and get splatted all the time, the disconnects, and all that. Try and find good times to play where you see an increase in the better players, playing. It may vary day to day and such, but still.
Take breaks, or you'll give yourself a headache like I'm sure most of us have gotten. Don't be too hard on yourself if you get a string of losses. One person can only do so much, and carrying a whole team is very straining. Team effort, check a lot to see whose splatted on what team and go from there. Just don't try and go in and Rambo it. I've seen a lot of people get splatted that way.
Also...try not to get distracted from the objective too much. Plenty of times, people have been far too distracted trying to splat others, instead of paying attention to the person on the Tower, with the RM, etc.