I saw Marlinpie's tutorial, and he said that he doesn't do the double box loop because it's inconsistent. He's right. Nothing about it requires weird execution like her flight combos, but there are too many ways to do parts too early or late so it's not really worth going for. I didn't see this anywhere on the Viper forums, so I think I fixed the combo with a simple change.
jS, LMHS xx L Seismo jump cancel H Burn Kick, MHS xx L Seismo, sj box dash H,S, MHS, jMMH double jump jMHS, L Seismo xx Burst Time
700,000+ damage, 910,000+ with DHC to Magnetic Tempest, 1,070,000+ with second DHC to Sphere Flame, works on the whole cast, works mid screen
This is just a standard BnB, so there's a lot of extra room for assists, EX moves, whatever.
Basically, by removing the first box dash and replacing it with H Burn Kick, they always fall so that the following M hits them at the right level for the next box dash, and this combo actually does slightly more damage while being easier to execute.