Too fast
To be clear, while Lex unleashed all his power, this attack was far from the strongest attack he could unleash. He had not even used any of his numerous affinities, nor had he used any Glyphs or arrays to empower this attack, nor had he used Domination, or even considered using Laws.
What he unleashed was simply the physical strength of his body, as well as his sword intent. Combined, his attack manifested in a bright white light that clashed against the electric beast, hundreds of times larger than Lex.
The attack and the lightning bolt, for a moment, forced each other to a standstill where they met, and the world around Lex suffered from its devastation. Everyone near Lex died, and those watching from afar felt like they had been struck by a mighty blow even though they were quite far.