Complex sentence: The Winx Club proceeds with a secondary plan to take it from the Trix who have now conquered Cloudtower and turned it into their base of operations but their Army of Decay quickly overpower the Winx Club and their specialist support and are once again are forced to retreat.
Split into simpler sentences: The Winx Club proceed with a secondary plan to take it from the Trix, who have now conquered Cloudtower and turned it into their base of operations. The Army of Darkness quickly overpower the Winx Club and their specialist support, and they are once again are forced to retreat.