This game is probably the best game to have ever come out on the PS3.

It kind of depresses me that one of your complaints is that those are the only good classes. Shocktroopers are great, Snipers are great, Lancers are great, Engineers are pretty okay. You certainly don't need to min-max anything, you can just play through with basically whatever you want, unless you really insist on super-high scores/times. Also, this game would actually be pretty bad if the AI did not cheat their statistics, because then the game would be trivially easy.