Take out the Krokomummy first. Then you can concentrate on the Prince.
The Prince is Fire-based. He's 60% resistant to Fire spells and takes 30% added damage from Ice spells. You'll want to put Fire Prisms in your deck,. as these convert your Fire magic to Ice magic, which does more damage.
The general strategy against Fire enemies is: Fire Prism, Fire Trap, Fireblade, Attack Spell. The Fire Prism should be used before the Fire Trap is, since traps are triggered in the opposite order they're placed, and you want the Fire Trap to trigger before it converts to Ice (otherwise the Prism converts to Ice and Ice magic won't trigger the Fire Prism). The Fireblade can be used at any turn. The total damage should be over 700.