Didn't know you had a hi-score thread for this game! Since it's my first time doing this on any forum besides mine, I'll clarify that I _never_ use more than one credit per play. It's the reason I always ignore hi-score threads; this is a rule which makes the labor way harder, and since apparently nobody but me follows it there's no actual competition.
So! I'd advocate a brand-new "disclaimer" heading every hi-score thread in order to know which is the case: true one-credit play or fake one-credit play (gotta love that naming convention).
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Can't remember my top score for this game, but I usually got to stage 4's "mid-boss". Stage 3's boss is indeed the easiest in the whole game and it's quite easy to figure out what to do so I believe it's better if I don't spoil it. Anyway, the screenshot from my forum's thread (which obviously doesn't show my actual progress):
(My score most likely sucks there since I am worried more about survival/clearing the game first.) Played in MAME, but at correct v-synch and native rez, so no actual discrepancies from the PCB.