Funnily enough big bull one of the hardest fights. he hits uber hard and there's no weaponry to help you out. A few tips from experience.
Kill 20 enemies beforehand in free roam. keep trying until you find weapons like the sniper or rocket launcher. To say they help platinum a tough stage is an understatement.
Jacks campaign is much harder than Leos. jack will face every enemy type while leo faces about half. and Leo is the faster more agile character making fights easy. jack is a grab and beat the shit out of them with a chainsaw type.
Leo is pretty easy to use. abuse that killer weapon lvl 2 (LT+Y). Jack is a little tougher. Launch them with a Y,X then while their midair do the combo x,x,x pause for a second x then LT+X,X,X,X. Check training and test the combos out.
Abuse the fuck out of stage items. you think snipers and R launchers are good? A tyre, sign, motorbike and all the others are just as lethal. They stun the ai forever even on hard and you can get off 2 basic x,x,x,x combos and then a level 2 which takes a lovely chunk of health off.
And of course defend and dodge like a little bitch. You are a little bitch on hard. 2 combos WILL end you. They take god knows how much. Don't forget to use throws. They can save you if you feel the shield is about to break and you cant roll away.
Finally for big bull? You'll love this. Knock him into the lava. he'll take a second to jump back up. Hit him as he jumps back on to the stage with Y+X. Keep doing it. Nothing he can do at all. It saved my ass when i was new and knew nothing of weapons and stuff. Gets you a nice platinum. bait him to the ledge sweep round even if hes defending keep hitting he'll fall in. Then hes at your mercy.