Magic users are the difficult ones to capture and absorb, but once completely swallowed up is when the fight is all over. The goo creature immediately absorbs the host’s magic essense, preventing from using it to escape.
 
Shining Armor is the stronger unicorns around, and his magic level skills are varied, useful for combat and defense like a royal guard.
 
I imagine the creature will gather the absorbed magic by building up a horn for itself grown from its forehead, mimicking the Changelings where its goo body came from.