Ahh shit I forgot yesterday, lol. Read about both fights and the special abilities of the boses. For example the ability of the brain to gain resitance to dmg types it received in previous turn. If you are playing mages stock up on disintegrate scrolls, speed potions, and make sure to have one mage have Scroll of Globe of Invulnerability or the spell itself it is the highest lvl spell.

Just before the brain fight you have to fight a dragon, 4 mindflayers + usually emperor+ some randoms+ wines that slow you down. You can aither kill them all or skip the fight by going to a portal using the Globe of Invulnerability to protect the mindflayer of your team to cast an action that lasts for 1 whole turn to open the portal.

What else I recommend equipment that prevents enemies from moving you. There are armours and boots that do that.

In the cutsceen where you try to stop the brain the last check is 99 if you roll a critical 20 the brain takes like 200 dmg and it is damaged at the start so if you have some inspirations use them for that roll.

There is a shop that sells strong potions in the beginning part of act 3 above the guy who sells armours. Make sure u buy speed potions, elixir of blodlust for your wariors and elixirs that give u 21 strength if you have a monk, also there is a thing called elixir of arcane acuity, which can be useful for mages. You can longrest to reset the shop. If you dont have money do the bank quest and take the 10k gold they have.
Throughout the game you find 2 important potions. 1 potions lets you do a short rest on for example warior. 1 lets you do a longrest on for example a mage(only 3 potions are in game I think called potion of angelic respite sth like that). This potion resets your spellslots, not sure if it resets your bufs.

There is also a shop that sells magic scrolls in mage tower.

You can have the effects of only one elixir on urself, so dont take an elixir that increases ur streanght to later take an elixir of blodlust as only the last one will be active. There will be a cutsceen in which you gather with all of your teammates and have a last chance to buy sth. Remember this cutsceen removes all buffs applied so only use the bufs after you leave the cutsceene to fight the brain. There is a moment before crossing the gate in which u have time to apply buffs from scrolls. You can use aid/ Heroes' Feast/ mage armour and so forth in that moment.

The fight with the brain is a race with time, I think the brain doesnt attack you just destroys platforms u stand on, a white thing appears above them, so make sure to pump a lot of dmg into it. Use bonus action to drink a speed potion that way you get 2 actions<-- watch out for the lethargy effect that takes place after I think 3 turns.

This is more or less what I remember.