One of the requirements for becoming Supreme Elite is to have 2 minis that have basic mini armor and 100 pieces of mini armor on them, each. This is the main reason that people armor their minis, as making minis battle each other is not an option yet.
Armoring minis is not a very common goal as it is a very expensive goal to try and complete, and you also need the Night Stone to be able to armor your minis. To access the Mini Armory, you will need to go to Mini City, and put the armor on your mini there.
Firstly, you need basic mini armor to put on your mini. The basic mini armor you need, has to have the name of the species you want to put the armor on. This costs 2500 kangabucks in Nothing Rare Here, but usually costs more in players swaps. They are very rare and very valuable items that are very hard to come by, not many players can afford to purchase even basic mini armor, not even most of the amateurs of Powerpets. After you have put the basic mini armor on your mini, you still have a long way to go. You now need to purchase mini armor for your pet, you have a choose of Empty Plastic Bottles or Empty Aluminum Cans, they both cost around the same price, not that far off from 5000 powerbucks. To put one piece of armor on your mini, it will take 360 minutes, which is 6 hours. A long time it may seem, and it is, but it will all pay off in the end :)
If you do all of the above, then you will have 2 minis with 100 armor on them, each, in no time at all.