I didn't bring out Flandre until the very last second, and she wasn't doing 10 million reliably until well after I hacked my skillpoints so that I could get her level 1000+ atk multiplier.
Right now, she's... level 501, with 1,320,224 reported attack (including +1020% from equipment, so only 60% less than 3 Gurthangs or whatever).
When I was trying to legitimately beat the left/right-hand bosses... I think she was doing 8mil unbuffed, if that? Or maybe less. I think I remember Master Spark doing some 6-8mil when his defenses were down...
Of course, it doesn't help that there's no thousands marker or anything in the damage numbers, so it's difficult to tell just how much ouch you're putting out. (=^w^)