I finally got round to completing the 15 minute melee last night. I did spend some of the match KO'ing the Alloys, but only till I knew if I KO'ed one more, they would bring reinforcements. After that, I used two different tactics, but both with the same results:
When the Beam Sword appeared, I went to the edge of the stage, anytime an Alloy got close, I swung the sword. I could have completed the 15 minute melee that way, but after a while, those trophy capsules do get tempting.
The other tactic I used I got the idea from some of the Brawls I have been in. When the Fan appeared, I grabbed it and again went to the edge of the stage, and then hammered "A" like crazy. The Alloys couldn't do a thing to stop me, and a few times I saw the damage meter on a few of them go up to 999%.
True, I could have just avoiding them for the full 15 minutes, but where is the fun in that?