If you are playing her for DPS, you’re missing the primary focus of the character. Yes, her DPS is low. Yes, she has trouble getting kills. She is classified as a ‘support’ character, not an attacker. You should have a lot more ‘assists’ then outright kills as Reyna. You should focus on keeping your allies or minions alive and protected over killing an enemy. Reyna alone is weak. You can’t use the overshield on yourself (unless you are playing solo PVE). Her basic weapons damage output are weaker then average. Her plasma pulse can get superior DPS for tearing down sheilds, but that will NOT get you kills, as the bonus damage is lost when dealing health damage.
And yeah, don’t go running after a fleeing enemy. that is a bad idea in general practice, let alone with a support character like Reyna. If you do want to get a wounded character out, get the homing helix mod, tag them with priority target, and fire off homing plasma pulse blasts. You get about six shots before overheat. If that isn’t enough to kill them, leave it. Better to go back to supporting your allies then getting yourself killed off on your own.

