While you may be able to see Seris through her stealth when you have her bounty, you can not shoot her while she is in her stealth.

Lex deals too much damage to fight on your own. If you're under attack from him, use Shadow Travel and move to a safer location, then regroup with your team.

Lex deals a ton of damage, super fast. Make sure you have a loadout where the cast time for Shadow Travel is reduced, because he can easily kill you while you're still casting.