Complex sentence: Together they manage to banish the Kraang back to Dimension X and free New York, only to contend with the Shredder's newly cemented control of the New York underworld and his efforts to create a mind control serum.
Split into simpler sentences: Together they manage to banish the Kraang back to Dimension X and free New York. Afterwards, the Turtles contend with the Shredder's newly cemented control of the New York underworld and his efforts to create a mind-control serum.