Now, whether or not Death Stalkers are too difficult to beat in general I can't say
The first fight I had with them, three of them in a smallish room, was ridiculous. I'm trying not to rely on savescumming, so I basically do the same thing every time I get to a door: hit Premeditation, let it tick to 5 seconds, enter turn-based, open the door. If it looks clear, walk in, select (but don't cast) Force Field, run it around to see if it indicates any occupied squares. Cancel FF, step back outside, throw molotov just inside, use Premeditation on a fireball of lightning, or if there's a large group of humans instead use the "I win" button (Premed, LoC, Bilocation) and drop the force field to lock them all in while an army of Doppelgangers kicks the crap out of the unfortunate burning souls. Anyway, barely managed the fight against the Death Stalkers, then forgot about the new debuff and used a health hypo, which immediately killed me
Tell you what, though, I waste *a lot* of molotovs trying to stay safe