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
using your transporter is a good way to avoid being killed too much, but taking advantage of the Lie In Wait card is good when fighting against Vivian or other similar characters. Just run and hide, use oppressor mines to sneak in damage, and heal.