Well, the obvious answer is just to not kill the other bosses.
They don't give very much experience, and if you can get to them you don't need to loot .
When I was grinding ramparts (60-65?-66) I started skipping the last two bosses even with a shaman group simply because they were not worth the time - Sometimes I would kill Omor, but Nazan/Rider is just a waste of time.