I had my full Rath set and a sonic bow 1. He is weak to thunder and para is easy to hit him with. Just dont get near him and wait for an opening, when he is in the water throw a sonic bomb to drag him out. Just keep in mind you should never be standing still. I rotate around him wait for an opening while charging then quick aim my shot and keep going. If you cant play online or dont do guild quest Id go with Hermitaur. With rath you can get 1 shot but once you know his 'tells' youll never get hit if you do it right.

Why do so many people have problems with him? I don't get it.  Anyway try makin the dragonwood LS it has pretty good raw damage and the paralyze attribute.  Create the ceanataur armour it gives good defense when lvled up and has the sharpening increase and strong sword skill. however you'll need to gem it with turtle jewels it has a horrible drawback of defense -30.  Ok teach your felyne status attack paralyze, abnormal status attack up, and affinity attack and he'll stun him along with your help (if you got the sword ^).

Bring 10 sonic bombs, 10-20 screamer and 10-20 gunpowder (for more sonic bombs), 10 potion, 10 mega potions, 10 honey (more Mpotion), shock and pitfall traps, (trap tool and genprey fang = shock, trap tool and net= pitfall), book of combos 1-4, and seeds if you want. ( if you want bring frogs to fish him out. if you do that he takes 100 points of damage

When you get him to jump out of the water quickly run up to him and if you get to him before he stands he'll do his tail whip 2 or 4 times (if you under his legs he can't hit you). When he does the hip check if you know how dodge roll through it or prey you can make it. When he does the beam run up and you can land 2 hits on his legs.  FLASH BOMBS DON'T WORK so don't try it.  memorize where he runs to jump in the water and put a shock trap (or pitfall whichever you want) and the next time he flees BAM theres some hits. Just do that and you'll be fine