It is trainer level/overall XP based, almost entirely.

The Elo score we have does behave like an Elo score once we get to Rank 7 where we get given it, but until then it's almost entirely based on trainer level.

It'll be an 'Elo number' bracket that you're placed in depending on your overall XP/ trainer level.

I had an 80+% winrate when I got given my Elo score, and got given ~1600. I'm level 34.

The matches very quickly turned into people who knew what they were doing once I steamrolled my first 2 sets of matches, and the matches I did lose were close and were due to bad luck (Bad lead matchup that cascaded into the rest of their team countering mine).

You can only beat who's been put in front of you and from what I've seen, it was impossible for me to get an Elo score of 3000 off the hop because of my overall XP.