Monsters are made from atoms, and atoms are mostly empty (99.9999999999996%).
All you need is a uh... Maxwell compensator which temporarily inhibits electromagnetic interaction. Then you can shrink any object to 10-13 of its original size.
Once the inhibitor is deactivated, objects will automatically grow to their original size again.
As a free bonus, this also prevents a living creaurecreature from suffocating (either because there is no air inside the ball or because oxygen atoms are too big) and from starving, since the process should reliably stop any kind of chemical reactions (including biological ones), thus basically "freezing" whatever object or creature you put in. No food nor air needed, and no aging.