First off, bring the Balanced Set of item.
Then bring an all dark team 5* and lvl 60+, then attack each enemy until they are almost dead, but be sure NOT to kill them. This doesn't hold true for the first battle though since they are all burnies. In that battle just use the Fujin Elixers to give 3 of your units full BB and unleash hell on the poor souls.
If you do this right you should always have the enemies on the brink of death with your BB gauges full.
Unleash all your units BB attacks. Be sure they are AoE attacks for maximum dmg and points.
I just finished up Frontier Hunter Mid and got ~520k.
I hope I can do similar on High but I always get killed. I want a Medulla Gem.