I'm no pro and this boss was also the last one I needed... Took a few tries to figure it out! You need to destroy:
* Both hands [10% each] (that shouldn't be a problem)
* Both cannons [10% each] = easiest part to miss! One of them shoots 3~5 ways (depending on your difficulty), the other one shoots straight lasers. The trick is: only one of them is hittable at a time. The boss needs to flip over (not turn, FLIP) for the second one to be shootable.
* Body (big rectangle part) [50%] = be careful! If you destroy this, only the core will be left... so destroy the hands and cannons before the body.
* Core (yellow sphere) [10%] = try not to destroy it before the rest, but that isn't really an issue
The rest is 0% destruction so don't worry about it!
Other bosses are pretty straight forward to 100% IMO.
I was thinking of making an achievement guide for the game... But I wanted to 1CC Arcade first, and it is now obvious -even with 10 lives in Very Easy- that I won't be achieving this before quite some time.
There's no way to "cheese" this game, you HAVE to get good and score big if you want to have a chance against the Last boss...