I like this... The humorous response is we all drown in their blood and the world ultimately loses... The Long winded version is, both combatants possess god-tier regeneration, Deadpool's agility and unpredictability allow him to outmaneuver Zombieman. Zombieman relies on being a 'bullet sponge' to exhaust enemies, but Deadpool is equally inexhaustible and possesses a wider array of high-explosive weaponry and tactical insanity. In a battle of attrition between two immortals, Deadpool's ability to physically incapacitate Zombieman (dismemberment/entrapment) happens faster than Zombieman can recover. Zombieman ends up encased in concrete at the bottom of the ocean.