I think I am actually sensing a darker plot-twist than we all anticipated. Something fishy is going on here, and I am not just talking Femto and his army of Apostles attacking at such a convenient time, rather, there are too many factors juggling itself right now for it to be played straight.
We have Sonia, Zodd, and Mule under Femto's command.
Zodd will probably have to confront Guts. Sonia will have to confront Shierke as per their prior meeting, and Mule is a wild card in all of this. Mule is somehow shocked at the fact that island is home to trolls! There's no way he is going to be played straight.
I forsee either one of the two conclusions to this arc:
1. (Really dark) Femto winds up invading and massacring the Elf Island and captures Casca after he/ Zodd has difficulties defeating Guts.
2. (Less dark, yet I buy this one more) Guts and the rest of his crew/ Elf Island escape Femto's raid, throwing him off completely. Maybe just the army dies and just maybeee the Elf Queen gets killed by Femto.