Find a Dunkleosteus in deep ocean areas, clear nearby predators, and use scuba gear or an underwater breathing aid. Lure it into a trap or kite it with a mount that allows you to use weapons, then fire tranquilizers into its body rather than its armored head; bring plenty of ammunition and backup weapons. Once unconscious, protect it and feed it suitable carnivore food while maintaining its torpor with narcotics or biotoxin.