Warhammer: End Times - Vermintide

Self explanatory, you must simply complete a mission with each of the game's five starting rotation characters.

Dwarf Ranger
Witch Hunter
Empire Soldier
Waywatcher
Bright Wizard

Now this can be done simply by joining a match in progress as well, so long as you are present (Alive or Dead) when the level ends and your team is victorious, otherwise some of the shorter standard missions include:

Horn Of Magnus - Prelude
Smugglers Run - Act I
Man the Ramparts - Act II
Well Watch - Act III

All of which can be ran in under 10-15 minutes and can be immediately replayed from the Inn.
As a note, Last Stand (Town/Fall) does not appear to count towards this.