I made this guide so that you would save your time, and not suffer like I did for 10 hours.
In this game, it is impossible, at least in an honest way, to raise the level above 20 in 12 waves by 1 difficulty.
Even if you manage to get LVL 20 on the last wave, it will not be credited to you, since it is necessary to finish the wave, not the game.

Download this save:
cloud*mail*ru/public/PRPf/SCexwF8f4 (you need to put dots instead of stars)
The saves are stored here:
C:\Users\*username*\AppData\LocalLow\GameSmith\ReMix

The game is rubbish. It's literally a piece of broken code from the main game.
There is a small chance that even with this save you will not score LVL 30, in my first playthrough with such a save, this happened.
My enemies stopped spawning and there was no one to kill.

The manual was done in a hurry, so for now, I will correct it soon.
If it's not difficult, support it with a like. I don't know why this is, but it will be nice to know that I helped someone.