Collin will throw his rock at the surface ekans and just have to trust Maia to be able to defend herself from the one burrowed with her.

ac 6 - (d20)[3]
physical normal - (d10+7)[9]

With his shift action, Collin will send out Rhea and then command her to rollout the snake. ac 4 - (d20)[9] physical rock - (d10+17)[24]