So far I've just been messing around with different characters and not trying incredibly hard.
I've gotten to 6 minutes left with Pit and my first try with him and down to 7 minutes left with DK.
My strategy with pit was to hang on the edge but the Green and Yellow alloys tend to knock you off. If you can manage to just get blues / reds left alive, I would highly recommend the ledge strategy.
It was frustrating after awhile because I couldn't get lucky with the spawns. I ended up just shooting arrows at 2 seperate alloys for a couple minutes and then finally they died and 5 spawned. Got unlucky with a bomb spawning and blowing everyone off the stage.
I used DK's Down + B move to slam the ground around me, it seemed to work very well.
In the end I think I'll just use pit and hang on the ledge. Worked the best for cruel brawl and seems to work the best for this too.
Do you guys know if you can complete the 15 minute brawl with 2 people playing? I did the endless brawl challenge with my friend and it didn't give us the completion until I did it solo.