Brawlhalla

Currently, there is only one legend that uses the greatsword. Jaeyun, who was added with the same update that added this achievement, is the only character with a greatsword at the time of writing this. He costs 7200 gold to unlock, and leveling him up to level 10 is the same as any other character in the game. Simply play whatever matches you like with him and he will reach level 10 after a few dozen of them.

EDIT: A new legend has been added, Mako, who also uses a greatsword, so there is another option now.