Here's my take on Metllao in this alternate earth with Supergirl: Kara has taken a much more aggressive stance in defending the planet than her cousin. Her and Flash and Bats have resolved several hot spots thus far but, while patrolling she knows about the war in Ukraine and without telling anyone goes there to fight. While wiping out the entire Russian military they unleash their secret soldier: Metallo who is the only survivor of the guards she killed while escaping
He shows up and it's her first exposure to green K. But, thinking he has the advantage Kara uses all her remaining strength to pick up a manhole cover and throws it right at Metallo's chest shattering the kryptonite!! He falls flat on his back with only his back up life support keeping him alive! Supergirl recovers and tells him "Good, I want you conscious so you can see for yourself.
She then uses her heat vision to turn him into scrap metal. He's just screaming the whole time. After she's done he's picked up. Back at the base they are about to repair him when Waller puts a stop to it. Amanda Waller: Over a billion dollars wasted on this worthless machine already that's enough. Unplug him. Last we see is Metallo's life support being turned off.
She blasted him with her heat vision and several pieces of his body armor are embedded in his heart they do what they all do with the various versions of Metallo they replace his heart with Kryptonite and turn him into a cyborg.