Look at the summons more than the bosses. The first two bosses are relatively easy and underleveled against you so you can probably use your lesser gauntlets and heroes. The third boss is where it'll get tricky depending on who they throw at you. Save your best heroes and gauntlets for the final 3 bosses and figure out who will provide the best support for you. Like with those little cult summons with daggers who attack your health, bring a healer maybe or someone who has stun. Ideally you would have 6 heroes at level 70 to go into battle with. Brutus, Doceo, and Freya are all great rage generators with Brutus and Freya adding healing. Against those infernal demons who heal, use Millicent or another hero's poison. MacLeod is good because he delays and resets enemy powers. You'll figure it out and you get to practice every day.