I know Trollager can sometimes spawn with a goblin riding it, but I want to suggest the ability for goblin to actively look for a Trollager, and ride on its back.
If the mounting goblin die and make the Trollager unmounted, and there is another goblin nearby, that goblin will take turn to ride the Trollager
If both goblin and Trollager detect a player, then goblin will prioritize getting on Trollager so that is can have extra brute force.