For me personally, survival is key. I engage when it is safe to do so, and my MP is high enough to keep me alive, and I back out when I'm being targetted heavily, or MP starts going down. You gotta know the dance, and how to shake people off of you, like cutting their line of sight, or going out of range so they switch targets and you can return to the fight or pop elixir.

I have gotten BH V on pretty much ALL jobs in this game. I know for a fact that all of them are capable of doing big numbers. Personally I put GNB and DNC as the worst. They have bad LB's that lock them in place and get them killed, or prevent them from using other abilities while they're going off. Or I just personally suck at playing them. Either way, I think it largely comes down to the player in control at the end of the day, no offense intended.

If I ever figured out how to post pictures here I'd share my results as well with pretty much any job to demonstrate that not only 500k+ is possible, but 1m+ is possible on pretty much all of them assuming you're A: not playing with, or against premades for a fair shot.