We have all tried killing the wyverns,and failed at least once,so heres one of the easiest ways to slay these beasts,what you need is a great sword with at least 300 attack power,a smoke bomb and any armour that can stand at least three hits from a wyvern,ok then,first of all,find an area where the wyvern comes and slay all the beasts in that area,that should be easy enough,two is better than one,so make sure you're in a situation where you're fighting only ONE enemy,now,when you find it get a good distance,and throw the smoke bomb at him,he'll be distracted with all the smoke,now's the time to strike

SPRINT up to the monster and hit the triangle button to execute a downward strike and remember,you can do dodge rolls while holding the great sword,so roll underneath the wyvern and keep using the side and backward slashes,they have no defense at all when you're underneath them,so if you get hit and roll away,just run straight back underneath him,this works for almost all of them except for the desert wyvern and a few others,try this in multiplayer,it will take seconds to slay a yiun kutku with 4 people using this technique!