Get the tail, then the mane.
After that, get the legs. It'll try to retreat in the water if it's limping.
Follow it, keep hitting it. It should retreat to area twelve.
Fight it one more time, it should die.
No, although using a trap would give you time to weaken it.
Spend a few minutes watching his attack pattern. Watch what he does before he does an attack. For example, before he does a sideways roll he shakes his mane and looks in the direction he's about to roll in.
I suggest doing the mission online first. It's much easier with other people and you can then buy the Ludroth armour set for extra water defence with the rewards you get.