I cant help you in the shaman-department but I'm beginning to get quite some BG-expiriences with my locks+priest.

Playing with friends will most definitivly help you out. I find that most times I lose a battle with pugs, its not usually about me not beeing enough effective, but all the pugs hanging around you doing nothing than wtf's and omg's

Anyway a few tricks I have that works for my group atleast:

EotS:
Start by getting the felreaver, then move on to mage tower. Just run straight into the mage tower and do the fight in there. Aoe is the boxers greates strenghts, but also greates weaknesses. If you can take the fight in to a tight area, this would usually be a good thing for the boxer since you here would be at equal grounds.(forcing the oposite team to also take a lot of aoe dmg)

WSG:
Mainly you should defend If one char dies, its fast to get him back on track since you're just by the grave yard.(however if the opposite team is also defending heavy, you might have to attack) Also do the fights in the tunnels if you can. as earlyer mentioned, tight spaces is usually good for boxers.

AB:
This is the hardest one imo. What I usually had success with here is focusing between two(maybe three) bases by patrolling in between them. I'm usually standing in the crossroads of BS, Farm and LM for quick access to all those three.

Also remember to pick your fights in EotS and AB. Running a suicide mission is seldom a good idea. When I know there are 5 or more enemies at one place I usually dont bother unless a lot of others are attacking as well. However when they are 3 or 4 I'm fairly certain that I'm going to kill them, so I focus on defending/capping bases thats not defended by more than that amount of players.