u/Adolom

This was the BBEG of my 6 year long campaign they won and I was using the chromatic greatwyrm stat block from Fizbinz. But they kept teleporting on his back to hit him. He could easy fling them off but I didn’t give them any disadvantages or anything for being on his back. Should I of? Would you have given them disadvantage on attack roll or make them roll to keep their footing?
I’m just looking for advice for future games where I might use this mini.
I was using a mini 2 sizes larger than gargantuan.