Complex sentence: Spidey and his allies then convince Mysterio to help them track down Octavius'base, only to find themselves pitted against the' new' Sinister Six in the form of the mind-controlled Avengers.
Split into simpler sentences: Spidey and his allies then convince Mysterio to help them track down Octavius'base. While they find the base, they also find themselves pitted against the' new' Sinister Six--in the form of the mind-controlled Avengers.