I just realized you can remove them super easily by just using Brick Separators 
As simple as it may seem, it actually genuinely works.
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I’ve found using flat pliers and gripping the top and bottom of the eye connection and pulling is the best way to get the brain piece off the head.
The best way is to put a axle through the mouth and pushing it against something immovable, like a table. If you want you can also put a liftarm that increases the surface area of the end of the axle, so you don’t risk scratching your table or something.