You will need to clear some of the requests from the Village Elder's Son.

the repel Ceadeus quest?

i think most of us just stay close to it and hit it till it dies ... (would be more convenient if u have sharp and fast weapon eg. SnS or LS or Slash Axe (sword mode)

at least that's what you should take advantage of bfor it goes to final area. 

u can take underwater balistas (in addition to the ones u can collect in final area) and use them to shoot/break the horn or just hurt it from afar. 

also can take advantage of the Dragonator when the monster gets near it.  if u can hit it early might even get a 2nd chance to reuse again.
sidenote:  if u run short of oxygen item u can also signal your companion with the Ancient mask ... that has ability to restore oxygen