When it comes to the world of Mega Man, MegaPhilX is like the gift that keeps on giving. Today he released version 1.2.2 of Mega Man Unlimited that saw a new option: Challenges. Challenges come in two flavors: perfect runs and omega robot masters.
If you're unfamiliar with the concept of perfect runs, it's essentially completing a robot master stage without taking damage and only using the default Arm Cannon/Mega Buster. If you want to see awesome perfect running in action, check out YouTube user RoahmMythril. Thankfully with Unlimited the challenge is just the robot master, but it's still buster only and one hit kills you.
As for the omega robot masters...oh boy. If you had trouble with the robot masters before (curse you Glue Man), the omega versions will give you more. Some of the robot masters will move and attack faster and their attacks will be intensified a bit (such as Rainbow Man Omega and Comet Woman Omega), while others might have the same attacks but hinder you in other ways (for instance, hitting Trinitro Man Omega on the bulb on his head is instant death to you).
If you're a fan of Mega Man challenges, this is definitely one to try. As of now I've only done a perfect run of Rainbow Man and only took down Jet Man Omega for those challenges. If you have the main robot masters down, these will give you a run for your money.