Also run into this. Only way to complete this bounty was to get my vectis, get to sniper position and shoot like crazy. Still it was a close one. As far as I can tell the counter does not care about how many enemies are around it falls at set speed and for every kill you get some % back like in sanctuary onslought. Which is bad when there are no enemies. I dont know how it worked before I avoided POE as much as possible, returned now for revenant.