Here are some tips for new players since some things arn't explained well in game. With scout get the ability on the left which allows you to target more ships at once as you will need this more later on than speed. With support get the shield reduce on the right and then get the heal increase 3-4 times then repeat. For heavy get the damage increase on the right. Although the crit on the left can be a little helpful you will find you can 1 hit most things if you go pure damage and usually don't need crit. Also I would suggest getting a few ship upgrade and then save for a support team mate with 27k+ distance. At this point make a few good runs and save them often as you will sale them much quicker than if you just leave them because newer ones show up more often for others.
I did it :) took over 1 and a half hours but I beat boss rush. And I have to say i take back my last comment saying they have too much hp. Unless you can beat them all in a row with 5x hp you'll never be able to kill the final boss. :)
Great game, the only thing that annoys me is the boss rush mode which is less of killing every boss in a row but more of can you last through 10 minutes of each bosses attacks without making a mistake. I can get past the first 2 bosses without getting hit but after that I lose patience. Please make it more realistic I doubt anyone can last through the 1 hour of dodging bullets for an unknown reward its already would be hard enough to fight each of the bosses one after another but to give them so much hp it takes 5 minutes to destroy the armor is unrealistic.
For those having trouble with the last 2 bosses just use plasma with splash and anti-mater bombs. If they are all maxed out you can just keep using the bombs to destroy the bullets and the boss will only have 1/4-1/5 life by the time to run out.