Here is my 100% Achievement guide and full walkthrough so you don't miss a thing.
My assumption is this would have been a lot harder without this BEAUTIFUL trick!
Before you begin, make sure your ships "Repusle O Tron" is FULLY upgraded!
This is important as with it maxed out to level 3 we can actually send back rockets with the tap of the
button.
So when the fight starts.. just move away from Robo Prez.
The only thing that will attack you is his rockets..
Pay attention as they can still do big damage to you.
Continue doing the same thing for the next 2 waves.. just move away, hit
until it completely dies.
Go to 1:12:45 in my part 2 video to see the beginning and all of this boss fight.
Hope this helps!
Part 1
Part 2