"Beat 250 players after they've beaten you in the same Deathmatch"
This mission should be changed. The level of gameplay in Deathmatch is the lowest from all competitive game rooms. Many times you encounter players using semievos or starter cards like Mac Hen or Coraille. In these conditions completing this mission seems impossible. (I have almost completed Demonstration of strength 230/250 but only managed 11/250 for Vengeful Rage)
The only solution i can think of is to forfeit every game until you meet the same player again. Having to resort to these kind of tactics certainly means the mission has a big problem.
If you have an opinion about this please write it here so the admins can take note.
Alot of times i play dm to work on other missions, so i'm still using a decent deck, but not a dominant one, i still win some doing this, i'm just not going 6-0 or 5-1 like with other decks i use when i'm going for the wins