(victory)
DM: And, at long last, combat is complete. You are the only "you"s left standing.
Fluttershy: I think I'm kind of combat-ed out now. Anyone else?
Rarity: Agreed.
Pinkie Pie: I dunno, I could go for a few more rounds of uninterrupted butt-kicking!
Rainbow Dash: Nah, I'm with Fluttershy. That was enough gimmicky combat for one session.
DM: Yeah, this concept needs a post-mortem. Thanks for sticking with it, though.
Applejack: Ah didn't want to bring this up in the middle, but why didn'tcha use the swarm rules?
DM: *...D'OH!*
Sometimes even a bad idea is fun once. I once ran a large scale battle where the players had to martial dozens of NPCs to defend 5 separate locations from simultaneous attack. They had players on several different maps in Roll20 communicating telepathically. I had to switch maps every time a new player's turn came up. It was epic, and huge, and took 4 sessions to finish. We all had a great time and agreed to never do it again :p
Unless you're doing a campaign path with set story checkpoints to level up to keep up with the pre-calculated encounters, it doesn't really matter when the players level up except that it should happen at a moderate pace. XP works just fine for that.
It's always struck me as an unnecessary extra complication to keep track of. Just tell people to level when you figure it's time to do so (so you can use new toys to oppose them). Keeps it nice and simple.
You know, it's in big fights like these that I'm surprised we've never seen Fluttershy use her healing abilities. I know that the Sentinel isn't as solid a Leader as Pinkie's Bard, but every little bit helps.