Can you say either le plus fort or le plus fortement. Both can be adverbs I think

Bonjour Diana ! Actually, I can't think of an example where "fortement" would be used with the superlative form in French. I guess it's because "fortement" is used only in specific context (-> Je vous conseille fortement = I recommend strongly) which don't really work with superlative. "Fort" being used to say "loudly" or "hard", it's more likely to be used in the superlative: Je serre fort. Je serre le plus fort. I squeeze tight, I squeeze the tightest. I hope that's helpful!