Well I will tell you how I see those classes If I had to go against them in PvP.
Agent: Usually has low HP but can do huge amounts of damage very Quickly.
Fixer: This is a profession I know little about, I rarely see these professions out at BS missioning.
MP: Great to mission or hunt with but in PvP I would put up my MKV shield and concentrate attack on the person not the pets.
Each has thier strenghts and weaknesses but I believe the Fixer to be weakest for the simple fact that very few people play this class and there must be a reason why but I would luv to hear from a person who plays this class, whats strengths and Weaknesses they possesse.
Hope this helps a little