a very effective counter to a shadowmane is a deadly tame being able to quickly deal with it,

another is a mount that you stay on even after dismounting or being dismounted such as a magmasaur. or a rex.

and another is just killing it from range with a gun. i recommend a shotgun. it has spread, good for emergencies when it attacks, for groups, and making quick work of them.

the last is another shadowmane. being able to sneak up to a sleeping shadowmane with a full charged bar and doing the super attack to make quick work. and the ranged for a stun and groups of em.