I consider the Mysidia Tower a great incentive to actually invest time on leveling up your spells.

Heal will be a life-saver a lot of the time since the bastards will keep throwing status crap at you, and the elemental spells will easily dispose of the Gigases.
Also make sure you level up Fire, Ice and Aero (aka Poison) to around LV8 or something and everything will be much easier.
Oh, and by the way, I kinda like letting Richard focus on Spears, because there are some nice spears in the tower and Richard is a Dragoon anyway.

EDIT:
Just discovered that you already beat the dungeon, so think of this as advice for next time.