For this, you WILL need:
A level 3 laboratory (or higher)
A Dark Barracks level 3 (or higher. Also, more than one is also recommended)
At least 4 Army Camps at at least level 6
A high rate of income from a Dark Elixir (I would suggest 2 drills)
The only troop you will need for this is the Valkyrie; fill your Army Camps with as many of them as possible.
With the laboratory, make 2 Rage Spells and 1 Healing Spell. (If you have a King Barb or an Archer Queen, use those as back up).
1. Scan the enemies base for a point for where most of their defences are grouped together. If they're not grouped together, look for the strongest defences and start there.
2. Before deploying any troops, place a Rage Spell trying to cover as many of the grouped defences as possible.
3. Immediately after placing the spell, deploy ALL of the Valkyrie's (yes, you read correctly, ALL OF THEM!), so that they will enter the spell area and attack the defences you previously picked out.
4. Once that group of defences is destroyed, the group should split into two. Place the second Rage Spell on the larger group and the Healing Spell on the smaller group. If they stay together, place both remaining spells on the group.
5. (Optional) A fair amount of your troops would have been killed, but only a few defences should be left, so if you feel the need to, deploy the King and/or Queen.
DONE! From this you should get 3 stars (or at least 2) everytime.
This is very expensive so I do highly recommend a large income of Dark Elixir.
Also, I have only tested this out a few times on players (All of which I won with 2 or 3 stars), however, I have also tested this strategy on the last few single player wars, again, all of which I won with 3 stars.