Attack his head. This will cause him to use his incredibly slow "thrust" attack, allowing you to run around him to his abdomen. Quickly use the Whirlwind and stun him. Then, use jump attack and Spin Attacks for maximum damage. Repeat until Stagnox starts to fly.
When Stagnox releases the Blastworms, hit them to turn them into balls. Then use the whirlwind to push them into Stagnox when he dives. Attack his now vulnerable abdomen with a jump attack and more Spin Attacks. Repeat this until he dies.
Force fead him your blade, and then gut him