Whats the easiest way of killing the yain ku ku and what item's I should..

That weapon is DROMENAIL, it's deadly when used right. Simply upgrade serpent fang (sns).

To kill kut-ku you'll need.
1-2 pitfall traps.
3 potions and all the first aid meds from the blue box. 1 sonic bomb
1 flash bomb
And 2 mini whet stones

Oops almost forgot to say how to kill him. Here are some tips from a guide I have.
When they become endangered they run away immediately, revealing a chance to attack
Their abilities are lower than normal, and they leave themselves open to attack a lot. Dizzied Yian Kut-Kus are completely defenseless, and you can attack them freely.
if a kut-ku hasn't spotted you, throw your sonic bomb near it. This will deal a huge amount of damage
An angered Yian Kut-Ku no longer is susceptible to explosive sounds. So don't bother trying.
Hang around their legs and attack from there, little damage can be recieved from that area. Also kut-ku is weak at the belly
Oh I forgot to say. Bring 2 paint balls.

Well in order to kill a Kut ku (very easy way, my way) get a gun ( doesnt matter if it's weak but get a strong one if you have (egarbalist(not so good), sandstorm+ or shotgun white or blood), flash bombs (maybe 2) potions herbs and first aids. Clust,crag,pellet,normal lv1
<---- I told you what you need. Know what you do is go to area 7 go up where the bug is and wait until it comes. Shoot clust first then crag then change the bullets. Heres an example how you do it: CLUST , CRAG , pellet , normal 00shots.
Shoot him until hes dead.