Bleed 2

For this solution, you need to have finished act 2 of the story mode to unlock the kitty chopper boss (thanks Y2Kasper10 for the addition).

From the main menu, I recommend loading up challenge mode with the following settings:
Character: Wryn (selected from the main menu)
Difficulty: easy
Boss A, B and C: kitty chopper
Weapons: Katana/pistols combo (selected from start menu in battle)
Air-dodge: Classic (selected from start menu in battle)

For this fight, you'll want to hang at the right side of the screen. The choppers will mostly stay on the far left of the screen, which makes it much easier to dodge and deflect the the choppers' projectiles. There are three attacks you'll especially need to watch out for:
- Sometimes a helicopter will fly directly at you on ground level. This attack can usually be deflected for a lot of damage, but if the chopper is yellow you'll need to double jump to evade it.
- The helicopters will occasionally shoot huge missiles which, again, can usually be deflected for a lot of damage. If you can't deflect it, your best option is to stay as close to the bottom right corner of the screen as possible. This is to avoid a huge car that will bounce into the screen when the missile hits a surface.
- Sometimes missiles will drop straight from above. This one is especially hard to avoid with everything that's going on, but jumping usually did the trick for me.

Good luck, and remember to use your slow often!

This method will also unlock the following: