jC, sC, f+B, f+A (1 hit), db fwd+BD, dp+C (393 dmg)
This is the max damage I've been able to get for 1 bar, simple and hits hard.
What's interesting about this combo is that unlike Takuma where you have to start charging immediately after doing the f+B, Robert can actually cue up the f+A after the f+B and start charging early.
In other words, you can enter the f+A early (right after entering f+B, you can even enter it right before the kick connects, its that lenient) and have it come out automatically. So you get lots of time to charge the db fwd+BD.