In a life-or-death situation, one mentor helped Supergirl survive an encounter with her deadliest supervillain. And no, it wasn't Superman that taught Kara her most valuable lesson but a hero who understands the value in being prepared for the worst-case scenarios.
Despite sharing the amazing power set as her cousin, Supergirl was nowhere near as prepared to become a hero when she first arrived on Earth. Kara Zor-El lived most of her life as an average girl on Krypton and was as powerless as everyone else when the planet was destroyed. But upon her arrival on Earth, Supergirl gained Superman's powers and instantly became one of the DCU's most powerful heroes. While Superman was thrilled to have a blood relative on Earth, Batman was initially a bit more distrusting of the young woman. However, he came around to the Kryptonian once he was certain Supergirl shared the moral compass of her cousin.
And it's a good thing the Dark Knight took to Kara, because without him, Kara may not have lived during one nail-biting fight with a villain. In Superman #40 by Sterling Gates and Jamal Igle, Kara has been charged with finding and detaining Reactron, the villain with a gold kryptonite heart. By setting up a decoy, Reactron is able to get the drop on Supergirl mid-flight. As the two tussle high above the Earth, Reactron activates the gold kryptonite in his chest cavity and temporarily depowers Supergirl. While plummeting to the ground, Kara thinks back to advice Batman gave her. He informed her that encountering villains like Reactron would be an inevitable, so she must be prepared. With Batman's advice to use what she has on hand to fight back in mind, Supergirl uses her cape to glide down safely onto a nearby rooftop.
Even though Supergirl has experienced gold kryptonite exposure before, it never has it put her life in danger like this. It's interesting that when Kara found herself powerless at the worst possible moment, she didn't think back to anything her cousin had told her, but rather Batman. If it weren't for a lesson in preparing for the unexpected, Supergirl might have met her death.
Few people in the DC Universe understand the seriousness of being without powers in a world filled with aliens, magic and scientific marvels as much as Bruce Wayne does. The only reason Batman has risen to the level he has in the Justice League is because he’s planned and prepared himself for most eventualities. But just because he's a powerless human doesn't mean that those with enhancements and abilities can't learn something from him. Being prepared in a world that runs on the unexpected has kept Bruce alive and he was able to pass that along to Kara. By being aware that her powers could give out at any time, she had the mental fortitude necessary to keep calm and think up a plan in such a dire situation. Batman would certainly be proud if he could see how Supergirl handled her first worst-case scenario.