On the griffon? I tried like 4 times by jumping off a rock that was above it and still caught nothing. And no, I only started spamming attacks after it felt like I was making no progress moving on top of the thing,.
I think it's a problem with directions, I usually never have any problems climbing from the tail or legs up to the center of the body. But have a hard time climbing from the center of the body to the head, might also be because the griffin is flailing heavly.
I had an awesome encounter on my 8th(ish) playthrough, my pawn called me over while the griffin were hanging in the air blasting waves of air at us.
Once I got close to her she launched me up in the air, and I managed to get a hold of the griffins leg up in the air.
I then proceeded to climb up to his back and repeatedly stab my fiery daggers into her until she caught fire, resulting in a fiery inferno tumbling into the ground.
I... freakin'... cannot... WAIT!