Use shield of faith to help keep the mind flayer alive.
Shield of Faith is good ...
Protection against Evil and Good is better tho, shame we can no longer make Shadowheart to prepare it.
There still are other ways ... and i believe there is even one scroll in prologue!!!
Anyway my personal way to cheese the s**t out of this final combat:
1) Take all explosive barrels you find.
2) Set them on the single side of entering corridor ... CAREFUL THO! There are fires, you dont want to light them too soon.
Also make sure you dont block the path, otherwise those two other cambions will spawn on the other side of room and your trap is wasted.
3) Cast SoF and PaEaG on Mind Flayer.
4) Wipe out all imps and hellpigs, position your party on rafts.
5) Just keep sniping until additional cambions come.
6) Blow the barrels. >:] This usualy takes them to at least half HP, sometimes even less ... my most sucesfull explosion left one with 3HP and killed the other.
7) Finish the General with Caustic Bulb(s).
Tada!
//Edit:
In worst case scenario, remember that he have only one attack per round ...
So if you have REALLY LOT OF TIME ... and patience ... and dont mind geting it all ridiculous ... you can easily defeat him by sheer numbers.
He kills one of yours > other of yours will "Help" them alive > another member of your party attacks > and repeat.
Its a long and tedious process, but its litteraly impossible to loose this way, even 1v2 is inevidable win for you.
