Climb Onto a Bigger Creature

Get into position.
Make a Athletics or Acrobatics check vs. the
target's Acrobatics check. If successful you now
move with the target and have adv. on attack rolls against it.
It's body counts as difficult terrain.
The creature's ability to attack you depends on
your location, and is left to the DM's discretion.
Dislodging you takes an action making a Athletics
check vs. your Athletics or Acrobatics check ( you choose). DMG p.271