Metallo's Kryptonite heart gives him strength and invulnerability to match Superman's, and his Kryptonite heart also allows him to severely weaken and eventually kill Superman and Superboy without even trying.
Recovering from his brain being shut down by Grodd. Metallo went to Jump City to rob a bank and steal exploding darts when he was confronted by the Titans. When the newest Titan, Superboy, charged at him, Metallo caught his wrist and twisted his wrist as he exposed his new Kryptonite heart. When Cyborg tried to help, Metallo knocked him into Beast Boy. Starfire then tried to kick him down to give Superboy time to recover, but she only ended up hurting herself. His Kryptonite heart's radiation was then blocked by Raven, so it couldn't supply him with power, or weaken Superboy. He was then punched into the sea.