Oh wow, I didn't know it was one costume per completion. Will wait for the store to open, you know with a setup like that they are going to be in there!
My bad. Just got through survival on Normal after being interrupted by ranked matches endlessly and it turns out you do unlock all the costume tied to that difficulty, meaning each character only needs to survive 100 matches total, not 300 like I originally thought.
In other words, beating survival on Normal with a character unlocks costume 4, 5 and 6 for that character in one run.
Sorry for the bad info. I'm really glad it's this way, at least. The input reading on the later levels gets a bit ridiculous and I was only on Normal.
Tip for anyone trying to unlock the costumes: Activate fight request with the 'battle confirmation' option set to 'ask'. If you're doing badly in a fight, you can hit start and enter the online battle. After fighting the online battle, you'll be returned to the start of the fight with your health and meter as they were when you started previously. Basically a way of cheesing continues out of survival mode.
Unfortunately, there's no way of forcing yourself to be matched with someone else, so sometimes you'll not be able to use this method due to matchmaking. Pausing also pauses matchmaking, too, so you can't just wait it out that way.