This time we have to deal with two bosses instead of one, the octagon and the heptagon, and on top of that there's one very important catch.
The octagon is a bigger and more powerful version of the original enemy, by itself it shouldn't be much of a problem... However the heptagon has two rather unpleasant qualities, first of all it is invulnerable, so just try to stay out of its way as much as possible, and if that wasn't enough it will go after you for a short while as soon as you land a shot on it or the octagon.
It's quite likely to get hit during the fight, by the octagon's bullets or the bosses themselves, the first advice is rather obvious, try to get to the fight with as much health as possible. Given the octagon is shooting quite often my second and more important advice would be to use a weapon that can go trough the bullets (like the shuriken), however in my case I opted for the explosive bullets. There are a few good things about the explosive bullets: you can shoot in bursts, they are quite powerful and also the explosions may help destroying the octagon's bullets in the process.