I have encountered the same problem as GroWLiTHeTR. After playing for a while I've just finished the third campaign and ready to start paradox mode. Instead of moving on I stayed a little longer in easy mode to finish the challenges. Now whenever I start a mission it gives me like 3 enemies and it quits so I can never ever finish the level. Tried with the other campaigns and the same thing happened. Please vote this up and GRoWLiTHeTR's post so Arkuni knows about the problem.
 "Obliterate Everything 3"){kind=icon}
 "Raze"){kind=icon}
 "Days of Monsters"){kind=icon}
 "Versus Umbra"){kind=icon}
 "Battalion: Skirmish"){kind=icon}