Bug: Upon crafting Treated Leather (Leather x3 + Alkaheist), the OK button doesn't work, so you can't exit the Crafting Succesful screen and are forced to quit the game.
Also, are Leather and Treated Leather involved in any crafting recipe?
Tips: The first skills you should buy are the combo and air combo extensions, together with uppercut. This is very useful to single out a enemy in a crowd. Fury punch and back kick are useful too. Needless to say, abuse roll against bosses (note that you can roll-cancel your combo).
Also: You can Powerup in the middle of a stage, which means that you can heal in a boss fight. Gun Arm upgrades help to prevent overheating, so use the Gun Arm to keep your combos up (you can, for example, uppercut an enemy and the fire diagonally and then fury punch him on the ground. Experiment!).
Finally, never lose hope, this game is HARD (even on easy) so don't mash your keyboard when a fat guy hits you five times with his tackle (learn air recovery instead).
There's a little bug(?) in the game, if you use a save point underwater, you lose the Aqualung spell you had previously casted. Not that important, but still...
@apocalis20: Actually, the .HACK saga (both teh first one and the G.U. one) uses this feature. And if you don't, well, you're pretty much screwed when starting the next game. They are both on PS2.
After your level is over nine thousand you can upgrade everything I think. Also, an arcade in an idle game? You're not idling if you play that. Unless it played itself... Try to include that in an update.