How do I get to level 0? It goes from 1 to 100 and vice versa.
I don't have the save anymore, clear the browser's cache. Not replaying this anymore.
You can actually change mission screens before you start the mission - on the mission briefing screen. If you loop over from the top you won't notice it (otherwise it would be too easy), but if you try and loop from the bottom to the top the mission 1 screen will change to the mission 0 screen which you can then start.
It is technically accessible from the start (just like the practice level), but it would be impossible to beat without sufficient upgrades.