The Transformed Animal Omni Schtick "Magic Drain" is intended for use by Transformed Animal PCs vs sorcerous / supernatural GMCs. (It is, after all, in the section with hero Schticks, not the section for Foe Schticks.)
The power as written says:
Magic Drain After a successful attack against a character with more than 0 Magic points, spend 1 Chi and 1 shot as an interrupt. Target loses 1 Magic point.
The problem, of course, is that GMC sorcerers don't have Magic points. Only PCs have Magic points.
Rather than just throw the Schtick out the window as useless, I'm considering House-Ruling it instead. Instead of zapping a non-existent GMC resource, I think I'll reword it as:
Magic Drain After a successful attack against a Sorcerer, Ghost or Supernatural Creature, spend 1 Chi and 1 shot as an interrupt. The target suffers 1 point of Impairment for the rest of the scene. If the target has Magic points, they also lose one Magic point.
You should probably make it a point of impairment until the next key frame and limit how many times it stacks. The chi cost will limit how often a single character does it but imagine you had a couple of such characters in a group. Trying to operate with 4 impairment is going to curtail fights quite badly.
Good point about it stacking, I can see how that could get out of hand, especially if 2+ PCs had access to it. It is a Transformed Animal Schtick, and I've just got 1 TA in my group, so I hadn't considered that.
Maybe I could rephrase this as:
Magic Drain After a successful attack against a Sorcerer, Ghost or Supernatural Creature, spend 1 Chi and 1 shot as an interrupt. The target loses one magic point. If they have run out of magic points (or didn't have magic points in the first place), they also suffer 1 point of Impairment until the end of the fight. No character can suffer from more than 2 points of Impairment from Magic Drain in the same scene, and any character with magic points cannot have them drained below 0 magic points remaining.