How soon do Mages become able to hit Headless Horseman

Thx for all the imput. Already using the Power Guard method. I have a friend with a fighter. We killed one Big Foot and he gained 2 lvls and my cleric gained a lvl and quite a bit. Killing Headless Horseman gives my cleric about 17% and the fighter 25%. Problem is I can only use this method when the other guy is on with his fighter, so I want to know when my cleric will be able to do dmg on its own.

Can't you just keep the PG mule dead so more EXP goes to the cleric?