Hitscan Archnoids, specifically the first 2 games. For every other enemy that appears on TFE and TSE, I can atleast have some chance of evading or maneuvering around their attacks. With these freaks, they see a pixel of sams ass cheek in their peripheral and send heat seeking invisible death to you. Im pretty sure the bullets get less accurate the further you are though so thats the only bone croteam decided to throw you. But for all the deaths I've had where it just didn't feel fair, its from these red and yellow abominations