I'm going with the Cazadors. I know in most of the areas the Cliff Racers come out of nowhere and swarm you. But at least they are easy to kill and even easier once you level up. But Cazadors on the other hand are just hell. They are fast and their attack and poison is strong. I know I was playing on very hard and I was around level 3, but I got rap3ed by a swarm in seconds. Hell even at level 35 I try to avoid them because I have trouble with them.