It sounds like the mimic slime's behaviour was altered a bit* from DM, in that game magic use and strength were independent of each other (magic was an attribute of the head, speed of the feet, strength of the arms and defense of the torso) so you could have a lightning-fast, giant-club-wielding master spellcaster. I found getting HP for it a bit hard though since it can only copy the HP enemies have, not go above it and my humans usually had way more HP than the regular enemies so my slime is currently a combination of healer and attack mage with a giant club, high defense and speed but still tends to die first in battle. Sounds like the difficulty remains intact too, in DM I did use status effect spells (which I usually avoid in RPGs) because enemies are so damn dangerous you don't want to let them get many attacks off.
*= Or the reviewer didn't care to describe it how it really works.