I got out of bronze and now I'm pushing plat. For bronze, just play what you're good at, team comps an coordination are rare so don't rely on them. I would main a DPS, a tank, and a healer as you'll need to flex as you go up, but focus on getting as good as you can get on a very small hero pool. I climbed out of bronze with a 48% win rate playing Winston and Zenyatta. TLDR; git gud.