This is "When Lightning strikes twice" by Wendy again, with the Super-Powered Lex Luthor: wonderful story, Wendy!
few moments for my search...

ok, I'm here

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He sensed rapidly that she was entirely serious in her intentions, and a quick flash of alarm and regret passed through him. He wasn't ready to die yet. He had plans; he had a vast and powerful empire to build. He had people to corrupt and governments to topple. The fun had barely started.

Yet if he had to die, this was at least a novel ending. Gunned down by a blonde-haired bimbo with one of his own weapons. How ironic.

"Any last requests" she asked.

He smiled; he had the perfect answer, and the perfect epitaph.

"Help, Superman?" he suggested sardonically.

A bright flash filled the prison cell with light, and Lex Luthor was no more.
Now it's up to you, FoLCs!

Simona smile