The Distinguished Service Medal, or DSM, can be earned in any campaign, but with varying degrees of difficulty. To earn the medal no player must be hit by either a Baneling or Beastlings. Hits taken from Blindlings do not jeopardize the medal. Also, if the "Ling" doesn't break the user's shields it will not register as a hit, and therefore not forfeit the medal. However, the only classes that could possibly have shields strong enough to absorb a hit of that strength without failing are the Demolition, and possibly the Assault, and Flamethrower with CP bonus, and quite possibly XS-4 Armor.
The medal can be incredibly difficult. On Nightmare, the infrequent "Banetrains" will contain several Banelings. As with all Medals, level 1 and 2 can be earned in Recruit and Normal, rank 3 and up must be earned in Nightmare difficulty.