As one of Lex's opponents, remember that if you kill Lex he will spawn with you as his target. you will also be at a streak of at least one with will give Lex an incentive to get revenge for more credits.
Every Champion that is on a killing streak will give their killer extra credits to spend. If you are on a streak SOMETIMES it may be worth jumping of the map before Lex can make you his retribution target and gain even more credits.
Against everyone, just keep strafing left and right to avoid damage (especially against hit scan champions). Then aim and shoot them dead. Keep practicing for better aim and feel.
Lex has the loudest footsteps in the game. Turning off music and using good headphones might help you pinpoint his current position if he's behind a wall.