If you are having trouble in a map then start it in Skirmish with no opponents. Learn where the best items of the map are and find out where they can be accessed from. Then try to devise a way of controlling it. If you can deny your enemies from say the quad damage or railgun then they will be in a lot of trouble.
Also on the final level unless Xaero is occupied with something else or dead try not to go for the BFG. He will squash you in it every time. Another thing about that is that after hitting the first speed ramp if you move bacwards you will miss the connecting bounce pad and land on the other platform. You can also move to the side after hitting the second pad and bypass the BFG ramp if it is closed and you will land on the platform you started on.