Select the Spin Zone level and set the enemies to Elite . You can fight whoever you wish but the easiest at this level are Cerberus or Reapers. If you have unlocked Wrex, take him and Liara versus Reapers and you will have an easy time.
Use previously mentioned tactics to get this challenge over with swiftly. The Spin Zone map is quite a small map so try and attack the spawn point with grenades to avoid the problem of enemies flanking you. Other than that, it should not pose you many problems.
Upon completion, you will be rewarded with +2 Reputation and 5000 Credits.
Little needs to be said here. Fight with Javik on any map against any faction. Completing this challenge will gain you credits.