He will suck with your pre-requisites.
Why is he intended to have battlemaster? He sucks with battlemaster. Nothing says he needs battlemaster.
Get Block and slash, challenge, and bullrush. Now he has tons of guard and he can deflect enemy attacks. Set all of these to preferred.
Go reaver all the way down, and go all the way down the left side of two-handed.
Ranged AI runs into melee and dies, dual-wield AI is super squishy and dies. Bull has guard, and heavy armor and high DPS. I'm starting to turn my opinion on him entirely. Hes one of the best AI characters.
I'm currently running a high crit party with darius, varric, and bull with me as their tank. Bull does just fine. Hell, he does more then fine. He runs in, makes a tremoundous mess of the battlefield, and if he gets hurt he is even stronger. Even if he dies I know he still returned the favor on the enemy ten fold.
Too many people rate their companions skills entirely on survival. Oh, my companion didn't die, therefore he/she is the best. Nevermind what actual contribution they made to the battle in the interum.