I've just realised that the rules for how creatures use magic isn't super clear in RotPL, so I just wanted to make a post to clarify.
It's super simple, actually. Like a PC has to roll on Mind, the creature rolls on their Target Number and reduces that number by the spell level. Magic points are reduced as normal (remember, HD+10 equals magic points). Creatures do not have to reduce their TN by the PC's level as they would with a regular attack.
I hope that helps with your game. If you're unfamiliar with Romance of the Perilous Land, you can pay what you want for the download.