I've managed to kill one in one fight, by starting the encounter out of its range, so that it will shoot but not stun me, and just shooting a bunch of regular bolts, as I haven't found the means to make electric bolts (or where to purchase them quite yet).
But I could be wrong, but when I was doing my hit-and-run, I'd fire a pyrokinesis shot at them, run and hide, and then refill at the shrooms. Next time I saw them (assuming it's the same one on the same patrol) they had max health, so I didn't have the means to whittle them down as I had initially planned.
EDIT: Actually, I've spotted one that's at half health, but I think that was from an earlier encounter where I threw the one grenade I'd found, and my fireball, but I can't say for sure whether or not he was the victim of my other plot.