Finally beat challenge 4. Aside from luck you have to manage both bots. Keep your shield outside the minimum range of the dragon and keep your sword farther away than your shield but within shooting range. It's really helpful if you can get all three bots moving uniformly across the screen in that formation and you get about 2-3 good seconds of shooting without having to manage anything. I will say I had a little luck but used mostly skill and strategy I picked up on as I played to beat it.
I made the mistake of investing in engine early. Bad idea. Get to 40 turret ASAP. Next get to at least 100 shield. Then put the rest in engine for when you get to the boss.
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