By way of Tony Ponce, word has it that Mega Man Unlimited has received a nifty little update update. Version 1.2.2 brings Challenge Mode and its subsequent modes, Omega Bosses and Perfect Robot Master battle.
In Omega battles, each boss is enveloped in a powerful aura that boosts their strength and speed. If you beat an Omega form, their weapon in the main game will have its energy consumption halved. Perfect Robot Master battkes, meanwhile, are self explanatory - beat each bot without getting hit, Mega Buster only.
Give it a whirl here.