Spoilers below concerning the last boss fight so read with caution.
Here Konami decide to ramp up the difficulty ten-fold and to make it extra difficult you can only use Tobias. None of the other characters, gone are your melee knights, your wizard & your archer. I got truly annihilated on my first attempt and turned off the game in disgust.
It was not till a year later when my youngest son started to play the game I thought let's go back and tackle that last boss fight again. On my first attempt again I died but then I clicked and restarted the fight again (after going through umpteen loading screens)
When the first set of minions appear you have to attack them but make sure you deal the final blow on them to convert them over to your side. You have two types, mages & attackers, when you have converted the mage minions you need to get them to continue performing healing rain on all your characters. Also you need to ensure that the skill divine power (increases your attack power) is always topped up, you can perform this spell.
When you have taken control of the first set of minions, you can then set out to tackle the next avatar enemy (you have three to attack) I would have the mages use the ice spell but only if none of my characters needed healing & then my attackers would go in and attack. After a few turns you can easily kill the first fire avatar. This then releases more minions for you to attack, again make sure you deal the final blow to convert them to your side.
Now you should have a large team of mages & attackers to control. Use the same strategy for the second fire avatar but ensure that no-one is standing close to the avatar because they can perform a devastating attack that will attack nearby players. I just stuck with my mages using ice spells & then went in and performed the final kill myself.
The important point to remember is that the mages are your important players, do not let them die. If you need to use up a whole turn on just healing spells, do so because you need the mages more than the attackers. Also ensure you make the final blow to any fire minion to convert them onto your side.
The last fire avatar is obviously more tougher but all I done was have all my mages case ice spells to take his health down & then dealt the final swing of my mighty sword to finish him.
Now sit back and enjoy the ending & then start the game again to get the alternative ending