Lex has more health than you. He has more damage and shoots faster than you. He has more combat-ready mobility to chase you down. His ult is suitable for duels. To beat a Lex shows how much more skilled than him you are. Good luck.
If your Max HP is 2000 or less, a single shot from Lex will put you within execute range. With a headshot this goes up to 3000HP, and With Death Hastens these numbers become 2800 and 4200 respecticely.