Play the game conservatively on Easy. I used the dwarf. You do not have to hold onto the magic once you get it, so feel free to use it any time you feel like it.
This will take you til at least the third level. You can usually hit the blue gnomes multiple times to get extra magic power-ups, so don't let them get away without giving you all the magic they have.
I didn't try this if I already had magic, so I'm not sure if being maxed on magic will allow you to earn the trophy but I'd recommend just using magic on the blue gnomes if nothing else.