For my 2s team we were gold league. Fought a team that says we were slightly favored to win, so we won that game and we got promoted to Platinum. Checked the profile of our opponent, they were silver team only.

So don't worry about winning better teams to get promotion. The system will promote you eventually as long as you play your games.

I've had friends who got promoted from Platinum to Diamond after losing 3 games to diamond players. Weird but true!