I didn't battle with The Blazing Glaciers during the past few days, so the person you fought wasn't me
There are many people on Pokémon Showdown!, as well as a couple on Pokémon Online who use my team, and since I made an RMT of it, it's only natural that many people would be using it nowadays.
Anyway, if you want tips from me about Ãœbers, you should read this PasteBin document I once wrote (it's very outdated though, and I no longer agree with some of the things I wrote there, but I guess it could still be used as a rough guide).
As for Deoxys-A, many people don't use it mainly because it's a one-dimensional offensive threat that cannot switch into anything (whereas many other Pokémon, such as Dialga, Palkia, Giratina-O, Ho-Oh and Arceus-Ghost, despite being immensely powerful offensive threats just like Deoxys-A itself, also greatly contribute to a team's defensive synergy by being able to switch into other Pokémon and counter/check them thanks to their typing, bulk, or a combination of the two), and also because of opportunity cost, since it has to compete with Deoxys-S for a team spot. Deoxys-A is still #21 in usage on Pokémon Showdown! last month though, which isn't too bad, especially when you consider that many other excellent Pokémon, such as Latios, Latias, Shaymin-S, Tyranitar, Terrakion and Reshiram have even lower usage than it.