Is it really that easy? Yes. Yes it is.
Bowser/King Koopa – Super Mario Bros.
The OG of bosses that could die in one hit would be King Koopa from Super Mario Bros. Thanks to a conveniently located axe behind him, you could collapse the bridge Koopa standing on, sinking him into the lava. Of course, this was usually before you learned the princess was in another castle so maybe Bowser got the last laugh after all.