Beorn kills the goblin general at the Battle of Five Armies.
Beorn arrives late in the battle in his bear form and attacks the goblin forces with overwhelming strength. He kills Bolg, the goblin leader, along with his bodyguards. Once Bolg falls, the goblin army loses its command and begins to break apart, allowing the other allied forces to press their advantage.
Beorn’s intervention is decisive because the battle has reached a dangerous turning point. Men, Elves and Dwarves are already fighting the goblins and Wargs, while the Eagles also join the struggle. His sudden arrival helps prevent the goblin host from overwhelming the defenders around the Lonely Mountain.
A common mix-up comes from the film trilogy, which expands and rearranges several battle events and introduces additional combatants. In Tolkien’s novel, the named goblin leader is Bolg, son of Azog, and Beorn is the figure who kills him. Bard kills Smaug earlier, while Dáin leads the Iron Hills dwarves; neither is the answer to this question.