"How do I stop that banshee from eating my face?" provided good strategies for dealing with Reaper Banshees. However I am curious if you can damage the Banshee when she is teleporting.
Yes and no. While she's flicking from one position to the other you can't hit her but when she pauses to change direction or continue, that's your time to strike.
They don't teleport as much once you drop their barriers, repeater weapons such as the assault rifle and SMG will make short work of the purple bar. Warp Ammo and some biotic abilities also will make quick work of it.
Once the shields are down, switch to hard hitting anti armor weapons and put the freak down for good.
Keeping in medium range where her shockwave and biotic warp could effect you if you weren't in cover is about where she'll stop teleporting.