If you know that youre weapon is going to hit an enemy, use ice block. Becuase there's a chance that you will die before your bullet fire will hit the enemy. (Only use it when you think and know that its going to hit them)
The legendary damage for your attack is a good choice as the frontlines are for most slow moving and thus you can deal insane damage while counter their damage with you illusions (you can also use them to disrupt the beam of torvald)
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.