After you are done with Ghirahim, go to the Sealed Grounds and be ready for yet another boss fight.
After you finish the fight, open up the gate of time, cross over and talk to Impa. Once all the cuscenes are done, head to Skyloft. In the academy, talk to Gaepora. When you're done with him, talk to Owlan in the office next door. Finish the spiral charge training and head to the Lumpy Pumpkin, an island southeast of Skyloft. Talk to the owner, then head for Thunderhead, the cloud ball where you learned all the songs. Land on the rainbow island to start another boss fight.
Bilocyte is the parasitic tentacle that has latched onto the sky god Levias and taken control of him. It fires noxious goo from its eye stalk, but you can repel the attack with your sword to launch it right back at the beast. In order to get in an attack against its central eye, however, you'll first need to deal with its defensive mechanisms - the fins on either side. By swinging your sword to the left or right, you can send back its goo attacks at different angles in order to direct them at the fins. Once the fins are out of the picture, use vertical sword strikes to send the balls of gunk straight back to the eye.