Ok theres a glitch for this quest. Go into the quest and head straight for area 7 (with the Forest Elder) and just wait there till the time runs out. It will say you have won and you will get items from the Rewards box, as well as 3500z (it says 4000, but its paying back the contract fee) . Now this is the real glitch part. When you go to do the quest again, he will be severely weakened, and you should be able to take him down in 10-15 hits, depending on your weapon.
As for fighting him without the glitch, I suggest using flash bombs, then planting a Pitfall trap under him while hes dazed. He will fall in, then plant 2 Lg Barrel Bombs on either side of him, and use a Sm Barrel Bomb to blow them up. This deals extreme damage, even to this guy. I managed to kill a Basarios within 5 hits after doing this.
If you can, bring the items needed (Nets and Trap Tools for Pitfalls, Gunpowder and Lg Barrels for bombs) to make another trap for him. WHATEVER YOU DO, DON'T TRY TRAPPING HIM WITHOUT DAZING HIM FIRST. It says he avoids traps, well, he does. In fact, I sometimes plant a trap just so I have a safe place to run to. ONLY PLACE THE TRAP UNDER HIM, AND ONLY WHEN HE IS DAZED. Otherwise he will attack you and most likely drain your health.
As joeliredblue said, you ahould bring a Water element sword. I suggest using a Greatsword such as Finblade, or one of its improvements.
If you are a good gunner, then lure him to area 2 or 4. They have useful ledges you can climb up to. Just stay away from the edges, he has a nasty habit of firing his fireballs at the ledge, blowing you back or sometimes off the ledge.
Anything else I can come up with, I'll put here for you to take a look at.
For anyone who doesn't know, I used to be Tree Afro.
You will come to understand fear as I have...
If you were me, you'd want to be you again...