The Hulks team up with Heimdall to take down Malekith the Accursed.
Skaar accidentally triggers Ragnarok and incites the ire of Asgard. Detained by the all-seeing, all-knowing guardian of the Bifrost, Heimdall, the Hulks must save Skaar and battle Thor. Soon the Heroes find they must put aside their differences to fight against Malekith and save the realm.—Jiilo_Kim
When Ragnarok has been started due to the trickery of Loki, Hulk ends up breaking Odin's law by saving Skaar. The Agents of S.M.A.S.H. must set aside their differences with Thor, Heimdall, and the Warriors Three when Malekith the Accursed and his Dark Elves threaten to destroy the Nine Realms.—Wikipedia