Great Serpent Blade is a good choice. i killed it using that my first time. My armor was (going from top to bottom on the equip list)
Hunter
Battle
Hunter
Hunter
Battle
This insures that you get a +10 HP boost and +10 defense boost when doing a quest i think (kinda rusty).
Using this and some pitfalls and bombs, i killed the rath with only 1 death. if necessary, use a decent lance and try to kill him. its stronger and a little faster.