To mount this Colossus you need to ride Agro and move quite a far way back, avoiding his flame projectiles. You need to line him up with a geyser on the floor, he will move towards you and onto the geyser. This will flip him.
When he is flipped, use the bow to hit the 2 soles of its feet, this will cause him to fully go on his back. Once here, use Agro and jump on his shell. You can also climb on his legs which have a ledge that you can stand on. You have to then make your way up to his head, go up the fur on his body and then his shell.
The only weak spot is on his head.