The Official Monster Hunter 3 Ultimate Topic

There's a Jhen quest at HR 5 High Rank.

There is...huh...I don't remember doing it at all, to be honest(I have done it).

High Rank village to be exact. I can repel him easily, but I'm like THIS CLOSE to killing him. I break one of his arms before the final showdown, and his horns break from a few ballista shots.

Eat for Felyne Bombardier, which I believe is the third cooking option with Meat + Dairy. It really helps out. Don't really bother with the Ballista shots too much until you run out of Cannon shots in the first Phase. Use the Ballista shots you have from the box on the horns, and then just spam Cannons when he's in range after you've broken both fin/back parts. When he tries to stab the boat, use the Ballista Binder to stop him, and drop an Anti-Dragon Bomb on each tusk when you can jump on to them. Combined with some Ballista Shots early on in Phase 2 when the Cannons cannot reach him, you should easily break both tusks. If they aren't by the time he gets close, a few well placed Cannon shots will break them.

In Phase 1, when he comes out of the sand and does the bloated stomach attack, you can hit the Hunting Gong to cancel him. In Phase 2, Gonging the same attack will knock him over and allow you to attack his mouth, which is his primary weakspot. Make sure you have HP when you do that though, because when he gets up you will get tossed for a lot.

You can save yourself from damage while on his back by jumping off of it early when he starts to shift his weight a lot, that means he's about to toss you off. If you're fast enough, you can even jump right back on to the boat without having to deal with getting back on the hard way.

When both tusks are broken in the second stage, you can go for his arms to break them with your weapon, however if you're worried about damage it might be best to spam Cannon shots until you run out first.