The best elemental type to use against Kulu-Ya-Ku is Water Weapons, which are very effective across the whole body. Alternatively, any other element also does decent additional damage if you don’t have a Water weapon to hand.
The Kulu-Ya-Ku is incredibly susceptible to Stun and Waterblight, so stock up on items to help here, or equip weapons with these effects too. Blast is also a great option if you have a weapon of this type, as you can deal significant extra damage with it.
Kulu-Ya-Ku can be found in Shrine Ruins, Sandy Plains, Flooded Forest, and Lava Caverns.