The machine gun eyes give you a massive damage bonus on ogres. Kill some of them first. The large ogre at the top is giving the witch a shield, so he needs to go before you try to damage the witch. Now depending on how things are going, you can kill an eye or two then take out the troublesome ogres in the arena, then concentrate on the big fella. Remember that once the buff runs out to get another eye down. Once the big guy goes throw everything you have at the witch. Snipers are great on the ogres. Swords smash the witch. You may not have time to run this more than twice if the big ogre returns. If you do fail you do not lose the rune. You just have to wait for the summoners exhaustion to expire.