I spend my entire Melty Blood Type Lumina release day evening labbing Red Arcued. The final challenge was by far the hardest of her combo trial. The trick is that her ↓↘→B is jump cancellable on hit, so you can jump up after it really early to fully charge the jumping C. Also you can pretty much ignore the noted delay after ↓C.
No idea if it is actually practical though. Looks cool at least.
Edit: On the matter of practicality, I found out later on that you don't want to do double jump combos with Red Arcueid. This is because you can only jump immediately after an air throw if you didn't use a double jump before it.
This is important for Red Arcueid, because it lets her lock the opponent down via a jumping ↓↙← A onion ring after the combo.