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If you're hoping for something more than just an achievement, like a bonus level, easter egg, secret ending or anything else, then you're gonna be horribly disappointed.

Beating all the levels without dying feels good though.
Yep, it feels good all right.
What do you expect¿? I mean, this days new games even old games gave you nothig by beating the game on the hardest difficult. And older games didnt have achievements XD. Anyway, the devs seems passionate about the game, so let them improve the game.
I can't get past 1st boss on Hardcore... this dude in his final form kills me with few attacks. How do you beat him?