I have a decent record against warlocks who are using an imp, voidwalker, or succumbus. But the felhunter really takes the cake.. It's really difficult to cast spells when the warlock is hitting you with bolts or wanding you while the felhunter gnaws away at you in melee.. AND the felhunter counters your spell when your casting bar for mind blast or vampiric touch makes it far enough. I don't want to waste a psychic scream on the felhunter alone.. I'd save it when i'm close to both the warlock and the felhunter but the lock keeping kiting me away when I run towards him.
Btw, I'm currently lvl 59 so I dunno how's the situation at lvl 70+.