Don't engage her in a fight when you're near pits. Her Disengage arrow can wish you an instant death.

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.

Slither cancels Hunter's Mark. Giving your position as a healer is bad, so it may be a good idea to slither away to get rid of it. You can still heal your teammates from a distance!

If enemy Talus isn't using Faustian Bargain legendary, after finding his rune, it's guaranteed he's going to teleport here after a couple of seconds. Use it to your advantage, when confronting him.