After completing one match, check the Combat Sim Controls in the Armax Combat Arena to read the following e-mail:
You will first need to purchase the "No Medi-gel" match modifier from the prize kiosks. It is a silver level prize. Then simply set a match against any enemy and difficulty with the modifier and complete the combat simulation. This challenge is not mutually exclusive with other arena challenges and can be completed in the same match as other challenges, e.g. Citadel Arena: Geth Challenge or Citadel Arena: Spin Zone Elite Challenge.
After completing it, check the Combat Sim Controls to read an e-mail from the fan who issued the challenge.