The easiest way to beat down a warlock in combat is to send a giant flying monster that's immune to acid and has high spell resistance (black or green dragons for example). The warlock has a lot of trouble against those.
Overall, I think the warlock needs to be designed to fit the party. The sensory based powers allow a warlock to fit that roll quite well. A warlock 9/Mindbender 1/Master of the Yuirwood 10 with darkvision, see through darkness, blindsense, dark foresignt, mindsight, and high spot, listen, survival, tracking is awesome. He takes the place of a ranger, yet has the damage output as well. Not to mention, he has Improved Uncanny Dodge, Charisma bonus to saves, can speak with animals at will, has good hp, and can open up portals all over the place. In my mind, the Master of the Yuirwood PrC is the best Warlock PrC available.