If Oda felt that inclined to give Greenbull a weapon, it should have been something more relatable to his powers; like a Sickle would have worked perfectly, it's a farmers tool used to cut various plants. The irony of him having a weapon that would in a way be against the very nature of his being would have been hilarious. But Also a tinge of terror, as he looms over his enemy looking like the grim reaper, the polar opposite of giving life, which is what his powers are in a way.