As you get higher it gets harder to level up, mostly because you play with people that are really good. I would suggest playing Team Objective for awhile, get another ranking up and you will start to get better and when your feeling that your doing good, try some slayer games. The most important thing is, play with a group of people that you know and will complement your playing style. Nothing is worse than getting a bunch of randoms that try to go rambo. I have 2-3 friends that i play with on Halo 3 and we rarely lose games mostly because i have played/known them for over 2 years. Double Team if you havnt tried it is also alot of fun and really easy to rank up in if you have a friend that you play well with.
Most of all though, you need to practice and get better yourself, dont rely on your team the whole time.
