Related
Page
2
Win the Red Gem in The Castle of the Lost King and obtain a vehicle that can cross water.
Page
7
Use the four Gems and the Time Machine to battle Mondain, destroying his Gem of Immortality and then defeating him.
Location
Mondain's Lair
Notes
Runs away from you, returns to human form if left alone long enough

Location
Mondain's Lair
Notes
Kill spell makes him stronger; casts psionic shock (damage), mind blaster (reduces all stats), magic missile (damage)
Mondain's spells don't appear to require him to fire them in the 8 main directions, which seems a bit unfair. Of course, it's pretty easy to beat him to a pulp despite that, so it's OK.

Location
Mondain's Lair
Notes
Kill spell makes him stronger; runs away from you
When Mondain is hurt enough, he turns into a Bat and starts running away. Leave him alone, and he will turn back. It can be frustrating to chase down the Bat, but it can be done. Keep up the fight and he will fall without casting another spell.