Very straight forward and easy enough to do on the 2nd level Containment.
Run around with some squaddies and lazer any viruses in your way, make sure that you upgrade to at least 4 squaddies to make the job a lot quicker through the start menu.
just be sure to shoot down the flying viruses with grenades, these may respawn a few times so just sit your squaddies nearby and wait for them to come back, i did notice that while you are collecting souls with your engineers they stopped spawning.
There are also some cleverly hidden virus swarms hidden on the back of a mountain on one of the islands, by switching the camera around you can send your squaddies in and clear them out.
If you comb the map and this still hasnt unlocked also check for big red eggs that the flying viruses may of dropped.