His escape ability can be annoying since it shrinks his hitbox making it harder for you to hit him with your hook, try to hook him before or after it. Lex can do allot of damage so careful not to be too low on hp.
Hook him when he does his high damage attack (default Q), it will stop it and not reset his cooldown, he might try to escape but if you hit the next 2 shots you should be golden.