Apex33 There is no double jump on hard mode unless you have a D-level or higher combo. Besides that, it's probally broken buttons. Also, there should be at least a slightly different ending for killing the final boss on hard.
To beat the final boss on hard, you need to jump over his head, attack him from behind, and repeat, if he uses a attack besides strike, dodge it, and if you push him off, run away from the edge.