Lex, while able to flank and hurt you, is a bit weak when using an oppressor loadout against him. Don't underestimate your own close range power and you should always watch your sides and back. Being highly aware of the map layout is also recommended
Lex is a flank champion, yes but he has a very restricted movement, and he can't move vertically, so he's easy to spot and easy to predict where he's coming from.
so get to a high ground, mine the entrance, and you should always have the upper hand.