Despite being some of the weaker enemies in the game, don't let this fool you as they're quite competent in dealing some serious damage. Enemies such as the Grims or Leapers like to gang up and attack in swarms and it's easy to get out-numbered.
Any of these in numbers will cause considerable annoyance to most players as the majority can all take considerable damage. Chameleons on the other hand pose a completely different threat, but if you watch out for the signs you'll generally be ok.
The bosses in Resistance 2 really help to enhance the sense of scale that the game promotes. Each pose a completely different challenge and defeating them definitely feels very rewarding.