I dont really like putting DPS on cannons. Both times I've cleared it,w e had 1 tank and 1 healer on cannons. Most tanks I've seen can grab everythign and pull it to the main boss. If anyone else does get aggro, they just pull it to the tank just fine. Sure, an extra tank will make the MT's job easier, but it's also slower and more likely to fail. With all DPS out DPSing, we've kill the main boss right after he broke the final ward.