For this one you'll need to finish the game in "Heroic Mode" which is pretty much hard more. The stages are harder to beat since enemies and obstacles deal way more damage than on normal mode.
The very last stage will be a small boss fight but it's honestly not that bad. The boss has two phases which have both quite low health. Try to make sure that you found a few if not all upgrades in the previous stages - you will need them at this point!