Stalk3rchief said:
Hate to ruin the stream of creative answers, but, why not just shoot them? Armor isn't bullet proof or even bullet resistant. Hell, arrows went through the suits.
Maybe a nice fixed gun, like a mounted 50.cal or an M60, with a rather large barrel to finish them all off.
Depends on the bullet, the range and specifically what the person means by "full plate mail", seeing as plate and mail were two seperate types of armor. I suspect this implies a late age (gothic period) knight, wearing a suit of interlocking plates of iron/steel with chainmail being used underneath to protect the vulnerable joints. This armor is actually surprisingly capable of withstanding bullet fire in the form of pistol fire, non slug shotgun rounds and the like.
The real problem is, with 15,000 of them, there is no single weapon that can reasonable keep them from closing and killing you if determined enough to stagger against the losses. Any real machine gun would overheat long before you ran out of targets and any of the heavier weapons have limited fields of fire through which they are readily aimed. Combine that with the simple fact that ammunition feed systems will only hold a limited number of rounds, you'll find that either you're surrounded and overwhelmed, the weapon malfunctions and you're overwhelmed or your are forced to reload and overwhelmed. Plus, thanks to the proximity being listed (no fat boy) you at best have a few seconds gap to play with from the outset.
That said, there is a solution to this problem, and it lies not in the realm of personal weapons. The most obvious choice is a field of claymore mines - they were afterall designed to stop this exact form of assault, and if the field were prepared in advance a single person is more than capable of operating a number of them. Armored vehicles are out of the question unfortunately since all of them require a crew of larger than one to be combat effective - both the M1A1/2 and M2 have a minimum crew size of three (and the loader in the M1 is going to be hard pressed to keep up the fire)