Wear flame-retardant suit... Your teammates will probably not like you.

I'd say that you might try staying on defense and just fearing the heck out of everyone who tries to go for the flag. You're gonna get owned by rogues after (or during) the first assault, but it might buy time for your team.

What works for me with 4xShaman is to go with the offense (not actually run the flag, mind you) and support my flag runner until we get near mid-field. Then I break off and go pop the Enemy Flag Carrier with insane burst damage to drop/return the flag. That will often score the first cap for my team. Subsequent points I either repeat that strategy, or I hang back on Defense and prevent the other team from grabbing the flag. This defense strategy is only working now that my guys have much more gear, however. (And it requires your teammates to actually mount an effective offense.)

5xWarlocks in WSG is going to suck. Just be prepared for that.