it happens, level 5 has same problem with level 11 that a level 11 has with a level 18
The trick is to start a fight with low level players, fight fast, do as much damage as possible before bigger players join. (usually) mages wait for it to be close to GB before they join, not always the case but most of the time.
If you want a level based fight, go chaotic/arena/temple etc.
Plateau is great fun.
No damage for those who care about that kinda thing
Double valour - never really gonna complain here
extra reps - again, great!
Just fight smarter and don't join a fight that's already a GB. Chances are, if you start the fight, and there are enough players there, it'll turn into a GB anyway and you'll all get even more valour even if you lose the fight.
Enjoy the event! ^_^