The key to the hard badge is to beat the intermediate bosses (at level 10, 20, 30). If you concentrate on buying only Energy Infuser upgrades until level 10, you should be able to beat that boss easily. This will give you 5 upgrade points, so you can get Antimatter Shield and two interest upgrades. Then save all your money until level 20, and buy as many Energy Infuser upgrades as possible before starting level 20. If you beat at least the first three intermediate bosses, you should be able to reach 20% interest and get most of the useful bonus upgrades. Good luck!
This is an average game. The hard badge is a bit of luck: if you don't get a few speedups in a row right in the beginning it is much easier. I scored 115 on my third try at infinite mode.
Not a very hard hard badge, but the game is fun. The bosses require timing - you have to hit them between their shots. If you keep them moving at a reasonable speed, this is even easier to do.
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