Ride around on Agro and fire your bow at the 3 different white sacks on its underbelly. Once all 3 are punctured, he will skim on the sand and you can jump on any of his 4 wings.
Once up on his back, the 3 weak spots are behind the flaps on the Colossi's back. He will knock you off when you destroy a weak spot, so you need to repeat this twice more.