Well...from what I've heard...
Assaulter is for those one on one situations where you can quickly ko a monster...
Thieves...it's basically weak..but it can be used to mass slightly weaker monsters..
And Pickpocket/Meso Explosion...It's kind of in the middle as you steal mesos and explode them which don't actually do alot unless you drop mesos...(You WILL get poor from overdosing Meso Explosion)