Metagaming honor mode:

Harpies:

Be sure to cast calm emotions on the party and sanctuary on Mirkon before the battle starts

Grym:

He always falls for minor illusion and you can lure him under the hammer. Use bows to activate lever and wheels.
Nere:

Protect yourself from charm. If you recruit the duegar and push him into the lava the battle is pretty easy but you can't remove his head.

Ending the curse:

Use wall of fire - not hunger of hardar - where the harper archers spawn. Be sure to prioritize them over any other enemy. Spirit guardians will take everyone out. Time for haste potions!

Oliver: space out your party before battle begins. Don't attack the shadow shield! Wall of fire will take out the shadow olivers.
Kethric:

Hate to say it because I hate explosives in DnD but grenades are your friend in this fight. This is a ranged battle. Have a cleric on the left hand side with spirit guardians - most of the necromites spawn there. Just run around and to kill necromites and keep casting guardian of faith on the avatar.

Use grenades, arrows and spells to take out avatar. Use the telekenesis bracers to rescue aylin when she falls. Don't be afraid to kill Aylin with a grenade. She'll just get up again.

I always have Wyll in this fight - make sure to summon the cambion.
Orin

Orin isn't that bad on honor mode. Just be sure to protect yourself from being yeeted. Minthara's boots and Kethric's armor help. Genie summons are great because they can thunderwave the chanters

Sharrans

I wait until level 12. Protect yourself from blindness - two helms, a ring and devil's sight helps. Insect plague left, insect plague right, ice storm center. Follow with sleep storm center. Summons take on Sharrans who approach from the left and right. Ignore Viconia.

If all goes well you will only have to deal with one or two front liners.

Heroes feast is good buff and will make Shadowheart immune to fear.

Going in as murder hobo makes the battle easier but you will have to kill nocturne. Don't use radiant spells!

Sarevok

The battle is much easier if you pass the persuasion checks to enter the tribunal. Use dismissal to take out the bhaalspawn ghosts. Don't kill them until Sarevok falls. The best time to dismiss is right after Sarevok has been hasted. Insect plague again.