It sounds like you're fighting the one at the end of Riya's quest. Most of us are probably thinking of the one in Kara's second quest. The fights are different and it would have been nice to know which one you were fighting.
Sentry Drones are robots, and are thus auto-stunned by
EMP Grenades, bought from Anno. You could also cheat in some EM Poppers. Normally, you can only buy them on Dhaal, but they should take down the Drones in 2 rounds at most.
If we're talking consumables, you could also look into
Shield Booster Mark II for an extra 100 Shields and
Healing Poultice for 75 health (once/fight). Both can be legitimately obtained at your point in the game. Vi in Gildenmere and Nenne on Uveto, respectively.