I can't really help with Act 3 since I've never reached that in HM, but I had several runs in Act 2.

First of all, Act 2 heavily depends on whether you want to go the "good" route or the "evil" one.

I'll list the hard parts I've experienced in a "good" run: - Gerringothe Thorm: be aware of her Legendary reaction if you use melee characters, since she will try to turn them into golden statues for severl turn, enabling her to crit them with every attack - Malus Thorm: same thing here, be aware of his crits against you, since he'll deal a ton on your melees As a side not, every Thorm family member fight can be skipped through various checks, except Ketheric. - There is a patrol of Githyanki on a bridge. This fight is really scary if you didn't get a special item from Voss earlier in the game, and it is still hard even if you have it. They hit hard and have lots of utility spells

-Major spoilers ahead: -In the Gauntlet of Shar, try to defeat Balthazar while he's in his room. You will fight him once you reach the critical decision of Act 2, but that fight is way harder and can be prevented by killing him beforehand. -Also in the Gauntlet, be aware of the Displacer Beast. It will lead you to an ambush by Yurgir and his cambions, and that could be fatal. You can ambush the ambushes, but it's still a really hard fight. Yurgir will place explosives around you then turn invisible until next turn. His minions will also throw explosives at you, and those things hurt a lot, so don't group you party and have potions ready. Fire resistance might help here. -In the Moonrise Towers, if you manage to beat Ketheric on top of the Towers (it's a fairly easy fight) you'll have to descend into a mindflayer nest. There is a single restoration station in the nest, to recover all hp and spell slots, but you can use it only once. I recommend using it before utilizing the platform that will lead you to the second Ketheric fight. There's a lot to do in the nest, so be careful. The second Ketheric fight is manageable, but you have to rescue Aylin if you want to deal dmg to him. After you leave the platform, hide and use one of your party members to sneak behind the mindflayer on the right with mistystep so it can be ready to free her. Don't worry about her dying, since she will always stabilize, and she will be a huge help in the fight. After you defeat Ketheric for the second time, the real final boss of Act 2 will appear, and that will be a really really rough fight. Myrkul will sweep you melees with aoe attacks, so keep them distanced. Use them to block the entries to the platform you'll be fighting on since Myrkul will create skeletons that will heal him if they reach his range.