The mad scientist should be the primary enemy, and he should be accompanied by a number (maybe three to five) of his creations. Mad scientist tosses debuffs around and supports his creations, which deal the actual damage. Maybe vary the creations some; you could have a tentacled grappler, a low-mobility acid-spitter (squishy but with high damage), a disruptive flier (maybe it has a tail that it swings beneath it to trip people), and a standard natural-weapons brute.

Make the mad scientist level 7, and the creations CR 4. It'll be a rather tough fight (EL 10), especially since the action economy is mostly even, but it shouldn't be horrible if you don't have the creations focus fire on one party member at a time.