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.
I wouldnt say that i'm
that bad at video games, i can normally get to the end of mission 2 on MS1 & 2 with 1 credit, but from there onwards the difficulty just seems to go through the roof. I dont have the attention span to play over something enough to memorize patterns. The only game i have done this with is Contra 3 on snes, its quite hard but for some reason everytime i died i really wanted to keep playing until i got the pattern down, it never got stale.
And early 80's games, i dont really play much of them, theres a few that i like but i never gamed in that era as a kid so i cant really relate to it. But back to metal slug, i still love it. It was the whole inspiration to build my cab, and the reason i stumbled upon MAME enthusiasts/cabinet building in the first place. Hail neo geo!