Try to predict Lex's auto aim ability and deflect it! This should be able to take Lex down to a low amount of health. If you aren't able to counter ti it will all come down to who has better aim.
Good aim will take you a long way on this champion. Learn his range. Always have an escape plan. Understand how to win every matchup (will come with time). Use your mobility!
dont try to hard to chase the Evie unless sh's very low and you have Kill to heal on. most of the time she'll lead you into her team when she runs away. when in a 1v1 gain height on her and force her ice block.