...Toffee has four fingers, which has notably been pointed out in-text. Four is death.
This seems like a tenuous connection, I'm sure ... but Toffee hasn't always had four fingers. One of them was chopped off, permanent-like, by a certain spell cast by Star Butterfly's mother, Moon. This spell was, notably, dark magic, and involved Moon pledging her soul to things dark, and deep, and destructive (although this could have just been flowery metaphor intended to focus the spell rather than something she literally did.)
Point is.
Toffee's entire plan for the first two seasons of Star Vs. is to corrupt magic, destroy the royal family, etc. But a pleasant side effect of his plan - presumably successful in this reality - is that he gets his finger back.
Four fingers. Four means death.
What if the spell yet lingers within his hand? Would that not qualify as a "chink" in his armor? A weakness, unknown even to him, residing within a single finger?
What if we figured out a way to reactivate that spell? Make it spread?
Felldrake could help with that. He's a master of dark magics, and Janna's got plenty of magical ability and motivation to take Toffee down. Plus, Felldrake was found in the Complex, which as you recall was the keyword that revealed that "Four is Death" to us in the first place ...
... I'm not saying this is definitely the answer, but even if it isn't, it lines up surprisingly well.