At first Bellum just sits in the purple pond in the middle of the room spitting out ooze.
Avoid the ooze and it will splat on the floor and sprout an eye. Defeating these oozlings
with your phantom sword or grappling hook. 
Use the grappling hook to tear off the pink splodges covering Bellum and then pull him
towards you when you have done that. Strike with your sword as many times as possible.

When enough damage is done, Bellum leaps out of the pool and positions himself on a hier
tier of the chamber. Follow him up using the staircase but be careful as he is still
shooting ooze. Circle the chamber, shooting the eyes on his tentacles so they turn purple.
He then drops down to the chamber below.

Repeat the same procedure as before so he goes back up and then shoot his tentacles once
again. This time though, the close occasionally and can't be hit when they're shut. He
then goes up to the hiest tier and you must shhot the tentacles AGAIN. He'll then drop
right down to the bottom.

Ciela is then revealed to be the Spirit of Time and Courage. Go down to the lowest floor
and Bellum is furiously circling his ooze pool. Ciela then releases a yellow orb, a
Phantom Sphere, which allows you to stop time for a limited amount. Wait 'til Bellum gets
near and press the icon at the bottom of the touch screen. Then draw a figure of eight and
your surroundings should turn grey. Hit Bellum's eye when time has stopped as many times
as possible and keep repeating this process until he is defeated. For now at least...