in both game modes it can be done before you need to defeat the boss on foot

Normal mode:
Kill the boss by riding a dino with an utility variant (other variants cannot dmg it)
You can also try to kill the godlike or higher tier npc of the utility group

NoTame mode:
Kill a wild godlike or higher tier Dino with a variant of the utility group, this also include the npc
(godlike and higher tier wild dinos, not talking about the npc, spawn only in the notame mode)

Thank you