You can half the damage of Behemoth King's Firaga and Ardor with Flame Shields, Blizzaga and White Breath with Ice Shields, nullify Thundaga with Rubber Suits, absorb Holy with White Mask or Sage Ring, and absorb Darkga with Demon Shields or Black Mask. You can also use buffs like Protect, Haste, and Bubble and you should win against him. With this strategy, I did it at level 50.

Anyways, you should always just play through the game and beat the final boss before you fight optional bosses. (SPOILER)Vayne is A LOT TOUGHER at level 45 than level 60+ for example. But the reason that the final boss can't be harder than say, Yiazmat is because then it would be too hard. Not all people might have to patience or willingness to actually level up and beat optional bosses, since they just want to beat the game, so they can't make the final boss ridiculously hard.

I would prefer the final boss to be the toughest though.