You will get an email from Armax Arena asking you to help them solve some anomalies with the scoreboard after you have purchased all available items, except allies. This unlocks the Unusual Scores challenge.
Your first task is to complete a match against Super Elite enemies on any map. Choose whatever combinations you feel comfortable with and complete the match. You will be rewarded with a Sniper Rifle High-Velocity Barrel and another email.
This time, they ask you to complete a match with all possible modifiers , including enhanced player damage. It is probably best to switch down a difficulty for this unless you really want a challenge. On a lower difficulty, the challenge shouldn’t be much of a problem.
Check your email after the match to acquire the Pistol Power Magnifier mod and have them tell you that they think they have isolated the problem but need you to do one more match just to be sure. Select the Spin Zone, Geth Elites and turn on enhanced enemy shields. Turn all the others off except enhanced player damage as this will help enormously for Insanity difficulty.