1. Speed. Speed all the way. Hit all the pads, and do all the BRs. Very important. I found that out when trying to beat Chengou on Venom. The lead AI was putting a 7 secs lead after half the first lap. When you hit all the speed pads, you get back to him up his ass at the last lap, so passing him is no trouble from there. So speed first, weapons later.
2. About weapons, on tracks with BRs like Chengou: absorb only and use when you don't need to absorb. Because you will be doing BRs primarily, and they take off 15%. When on a track with no BRs though, fire, always (Except when it's mines or Bomb and you're in eight, that is common sense). Absorb when you only get hit.
Also, if you get hit more than two times, you need to restart. Unless you are really lucky you will not be able to get first place.
If you respect these two tips, you'll see it becomes much easier.
Remember: Speed first, weapons later.