The entirety of TTK raid bosses are bullet sponge dps races locked behind recycled raid mechanics that keep you from progressing if a single person screws up, the game bugs, or latency from a single fireteam member screws the rest of the team...so in all reality, all classes are essential. Nightstalkers and Defenders are needed for the insane dps boost and orb generation. Warlocks for dps and to kill trash mobs. Decent machine gun and sniper are pretty much a necessity for bosses too...rocket launcher if you are on trash mob control...but thats all pve stuff...with the new sub classes, vanilla raids are pretty much easy mode...as for pvp...that's another post...