Heya, been playing this game for a while, and I've found a fairly simple way to win, IMO.

Tired of being a squishy mage? Let your crystal do all the work for you!  So far in all my game experience for Mage, I honestly have never used the most left tree, out of the understanding that im am an MP hog, and hate seeing mp dissappear from magic atks, and prefer to watch my MP go the furthest distance it can take me.  So I hide myself in the tunnels and watch my pets maul everything in sight!

Skiil 1- Confuse(used often at start for my temp pets)

Skill 2- Pet(Hid behind them)

Skill 3- Tele(Running from multiple enemies)

skill 4- Heal(Tele from harm>heal)

Skill ^- stronger heals/pets depends.

Get lucky with books, you'll be happy.

The most  potions I may expend occur at boss fights, and in scenarios where I find myself surrounded, (unable to tele).

I always grab leather, and boost it up for all its evasions worth.

Used my wand everytime, even for satan.

Satan-  Killed him in 15+ turns…

Summoned 3 orbs in his face, and watched them rip him apart.

Put priorities from my own atks to the pets, if they appeared.  I get hit for roughly 20damage from unholy fire.  And a simple save is Healing spell, which at max, goes for 12hp regen for a few turns.  I stood in front of Satan where the dialouge left off the entire time ^^

Happy dungen diving, hope this helps ppl some.