well that was interesting. i actually had to refresh a couple of times for it to let me buy upgrades, then as i played (and finally broke the 10 second mark) i felt it was very unpolished.
"combos" as you say are actually actions interrupting each other, objects spawn sometimes right next to each other, and if you get going fast enough then these two problems come together and hitting the buttons means its an endless combo of starting kills, then disappearing people, then starting the next.
i'm sure this game would've been awesome! if... it worked. which it seems not to have worked for a lot of people.