Normally I hate linguistic perscriptivism but in this case, it honestly makes sense. You can't have a word that means both one thing and its exact opposite when used in the exact same phrasing/context and not cause confusion. It's literally stupid, and figuratively the worst.
In this case, it is demonstrably false that Sonicfox is unstoppable, since he has in fact been stopped before. To state that literally is being used as hyperbole only obfuscates the meaning of the phrase and serves to introduce confusion. "Sonicfox is practically unstoppable", "Sonicfox is basically unstoppable", "Sonicfox is all but unstoppable" are several alternatives which avoid any confusion. It doesn't matter if the technical definition of literally can mean figurative, as grammar and vocabulary are meant to ease understanding. If there is the capacity for obvious misinterpretation to occur, then I'd argue that the author hasn't done their job as an effective communicator and so this should be pointed out in order to be rectified (especially if viable alternatives exist which completely side step the issue entirely).