Scorn. Not because they're difficult, but because they will often spam their phasewalk that makes them impossible to hit for obnoxious periods of time. It gets so bad that it can mess with Gambit enemy spawns. Taken can be annoying in Grandmasters, but at least anti-barrier rounds can pierce shields and bubbles. Taken Knights can suck it, though. That flame puke spam is infuriating.