The proper way to win the final battle (which is separated in 2 phases) is to focus entirely on the Bellower.
The key problem in this fight is that the Bellower regenerates his armour and power every time his turn is up. In the first fight, focus on bringing his armor to 0, then shoot the special arrow with Alette or Rook, depending on your choices prior to the fight. If you kill other enemies, then the Bellower's turn will come more often, meaning the fight will drag on and on and will get much harder. However, if during a character's turn you cannot attack the Bellower, then your second best move is to weaken another enemy's power. Note that this phase ends after you have attacked him with the special arrow.
In the second phase, every time you attack the Bellower, he moves right after. So make sure you can break his armor enough so that you can quickly weaken his power to make sure he doesn't one-shot all your characters. Again, focus on the Bellower, once you kill him, the fight ends.
The only time I completed the game, my party consisted of Rook, Alette, Hakon, Iver, Griss and Eyvind, if I'm not mistaken. I made sure I had enough armor to withstand a few attacks, and Eyvind was there to restore armor if needed. I think I could have replaced Eyvind with a character who can deal high armor damage, as I didn't use him that much. This was done on normal difficulty.