i'm pretty sure the game was designed to eat quarters (or $1's in Australia). When you get to the later bosses it seems almost impossible to avoid some of the attacks. A guy i worked with claimed he did MS1 with 10 credits, even that is near impossible
Hahaha I don't mean to sound negative but is it possible you're incredibly bad at video games? MS1 is by no means a hard game, if you keep playing it you'll learn how to evade all of the enemies' attacks quickly. Just find out what the end bosses' patterns are and you'll 1cc it soon.
If you want brutal, quick shooting action (albeit of the shmup variant) down-
buy DoDonPachi and try to get to the 4th stage without dying. It will take you months. And still this is considered a walk in the park in comparison to its sequel or most other games Cave has made.
Btw are most arcade classics from the first half of the 80s even playable for you? They're all much much harder than Metal Slug... anyway, it honestly wasn't my intention to rip on you, I just wanted to put a couple of things into perspective.
MSX, which, afaik is just MS2 with some slowdown/bug fixes
This is incorrect. It does have those, but there are also important changes to the gameplay (enemy placement, some of the bosses appear at different points, refined gameplay); it's just that teeny bit better than MS2.