Failed jhen, again and again. My brachy LS and 650 attack isn't enough apparently. Of course I break all his part and use the gong at the right time.
Ugh, this fight makes me hate this game >.<
Try a dragon or ice weapon. Make sure you're using the ballista binder and dragonator as well as the gong.
My strategy using only low end gear: get felyne bombardier and optionally the artillery skill. Ignore the ballista shots in the box at the start, but grab the binder and bombs.
Gather 10 ballista shots, use them on whatever (tusk or arm), jump on the back and place two anti dragon bombs on the right tusk. Then do what you can to one of the two weak points on his back before he throws you off (let that happen).
Note: don't bomb the weak spots. Bombs and ballistas do FIXED damage, I'm saying to attack the tusk and such with those because you'll want to break them for drops.
when you climb back in to the ship, grab 10 ballista shots from the chest. Go back up and use them, use the gong or binder to stop his head on charges when he surfaces on the port side. Jump on his tusks when he lowers them and hit the weak spots - they should break by this point. Farcaster back if you don't get thrown off. Take the last 10 ballista shots out.
After this, dragonator him when he charges from the front. This is pretty important so make sure you get it figured out, you didn't specify if you were doing this so just making sure. The correct timing is hitting it after he dives down in front of the boat but before he surfaces.
From here, he'll attack from the starboard side one last time. Use your 10 ballista shots if you haven't, then gather the last 10 from the pile. In this phase I like to try and break his left arm with bombs and ballista shots, if you haven't finished off his back weak points you can do that too. Just make sure you've used all the chest ballista shots and all the ones from the pile. Save a gong or binder for the end of phase 1 because he sometimes charges one last time from the port side.
Phase 2, you get 20 more ballista shots in the pile. Gong him when he tries the big bellyflop attack and he'll fall down letting you get in his mouth and do HUGE damage. He also falls down if you stagger his belly twice, but you can't just stand in front of it and attack. I attack his belly by standing in his armpit.
He'll fall down briefly if you break his tusks, and he'll back up a LOT when his arms get broken. Staggering the belly makes him back up, as does the ballista binder. Of course you'll want to dragonator him early on and again if he's still not dead by the end. Sometimes he'll brain fart and bellyflop over and over while out of range of the boat, which is awesome because the boat will be completely safe whether you did enough damage to kill or not.
If you do all this with a half decent dragon/ice weapon he'll die by 5 minutes remaining. At worst you'll get a repel. If your weapon is really good you can ignore the gimmicks and just attack his arm and tusks instead of firing ballista shots.