Here's how I did it, with each match lasting about 3min 30seconds.
Map: Altar of Blood
AI: Brutal Drowning Doom
You: Drowning Doom
Start the match, immediately fly to the center and build 1 bride, 2 gravedigger units. Play the Rally Flag solo near to the right hand Fan Leech.
Now, Double team with the bride, run down the left hand side of the map and place yourself in the little corner on the left hand side of the stage. (Between the stage barriers and the stage building). Start spamming lightning using the bride's double team. Every now and again, you might want to build as many gravedigger units as possible (although the enemy won't overrun you before you destroy their stage).
The AI will not harm you, in fact, the AI Ophelia may decide to just stand there and watch you destroy the stage.
Easy as cake.