This is super hard to get to without savestates and rewind.
I've gotten close but not quite on the arcade version.
The NES port has nearly identical gameplay, all the secrets, but less graphics.
The hidden flags are there, and so are the underground silos, etc.
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You should watch a longplay and memorize where the bosses, enemies come out.
There are great strategies to bomb certain spots on 'area 16', so you wont be
trapped by shots.
Once you reach the point I show in the video below, thats it, and the the game
just repeats most of the levels again. You will see 2 large circular bases
shooting at you. Once you clear those, you see a forest section.
This is in the first 20 seconds of the video below.
Sorry, this Game Clip has been removed