Ok I figured out a trick that will help you kill the cannons. What you do is zig zag your way up to the wall. Hug the wall on the side of one cannon. This will give you los to kill one. Then move to the other side to kill the other. It seems to be working ok.

Defending the base I have found standing guard at the final wall is pretty good. If you can take out the vehicles alliance have to drive all the way back up. By that time you should respawn if you die. Just pray your at the same graveyard. If your not at the same grave yard start auto running your lost toons towards your main group.