Is there any way to cheese these fuckers? I'm trying the no mercenaries achievement.

I shoot first one, but I can't down him even if Rekarth and MC (water kineticist) both hit. After that another spider zoom-zooms out of darkness into melee range. It always does it, they don't patrol.
My MC is aasimar so I have invisibility, but the fuckers can see me for some reason.
i don't want to save scum until Rekarth finally crits the first spider with decent damage.