I've never gotten on Lao's back. Tried a few times but I just slid off.
I went on his back and it works fine. To do it, go on the bridge (area 2 or 4) and where there's a big gap on his back (or the big spike) jump then and your man will stay in position and you can carve him. But after a while, Lao roars and you go flying off You can get Hard monster bones and that off the Lao in for Hr3 so it can help people like me or domination for better weapons if we can't kill him