Spartan 117 & The Arbiter vs Gravemind
Most liklely the final battle of Halo 3.
The Master Chief has formed an alliance with the covenant Arbiter to put an end to the Gravemind and the flood.
Both warriors are armed with every weapon & gadget found in the Haloverse except the Halos. They have 30 minutes to destroy the flood's conscience before he assimilates with Cortana and manages to take control of Earth & Covenant forces.
Battle takes place in the Ark.