And thank you for giving me your toughts about that idea
You didn't know?
If you do the right actions in the right order, you can have Solaire of Astora summoned for the final boss. And since he can take aggro, it makes that fight really easy.
You need to join the chaos sister's covenent, and give her 30 humanities to reach the second rank (you will also get a pyromancy I think).
Then you need to kill the pyro fatass demon, the one that drops some catalyst. Once you killed him and reached second rank of the covenent, you will be able to open the shortcut you normally open at the end of the Lost Isalith. Nothing complicated, just interact with the door. It will open on that large corridor leading to the bridge kept by the titanite demon (that demon is the only one in the game that respawn, and he has a 100% drop rate of 2 special titanites, perfect for farming and upgrading boss weapons). On the way to the demons you have all the little parasite beasts, normally you only have to kill the shiny one (he drops the sunlight maggot) but in doubt kill them all. You need to do all this BEFORE you kill the lava monster boss (he drops the ring that reduces lava damage).
If you did all this right, Solaire will not go hollow (you also won't loot him, so this is best done in NG+), he will sit there instead, wondering what to do next (the same dialog lines he normaly has when he awaits you at the bondfire after killing the lava beast). You know if you did it right just after killing that boss, because he will not wait at the bondfire, maybe to make this work you need to summon him before the boss fight, I'm not sure I didn't test it, I summon him all the time.
Voilà! If you knew this already, then my bad, and if not, then have a nice playthrouw and an epic final boss fight.