@HerbieG totally correct in everything you're saying. Indeed the game was pretty random if it'd "count" various things or not and many times the "correct" thing to do wouldn't "count". Try it again another 50 times and maybe it'd work one of the times. (Which was very necessary to get *all* medals of the game, by the way)
On the last level, the bricks indeed don't always kill that final robot but sometimes they do. I'd say they kill him about one in six times or so. Also hilarious you can get the "Finish the game" medal by making it to the last level but then killing the princess rather than the robots. But you should note the medal does say "Finish the game" not "Beat the game" so maybe you're tired of that frog princess by then so you finished the game by killing her. She's finished. Done for.
Also do note you don't have all the medals but that final medal was likely not publicly visible at time you wrote the review, and now 0 points medals (those not earned by anyone yet) are visible, which is why I was able to see it, go for it, and become the first to earn it and now it's no longer 0 points so go get 'em tiger.
Medal description is correct. You do only need 3 stars in 30 of the 32 levels, which is good because I believe it is impossible to do in level 15 and level 31. For the rest you have to get insanely lucky and hope it "counts" what you want it to "count", with level 18 being particularly troublesome but ultimately was my 30th level 3 starring, so medal get.