We are aware of a possible issue with the mission "Mighty Morphin" where players are unable to complete the mission.
An update resolving this has been released on PC (http://support.gearboxsoftware.com/entries/22126761-pc-update-infor...) and will be coming to PlayStation 3 and Xbox 360 in the future.
In the meantime, you may be able to complete the mission by reloading the game or by joining a game where the host has not completed this objective.
Keep an eye on this page for any additional details moving forward.