Rhein w/out support is tough and mostly useless.. Doesn't matter how many hp you have if the other team just focuses you down... If I'm getting smeared as Rhein I will usually switch to Dva for her range and mobility... As Rhein, if you are the sole tank about the only chance you have is with a Mercy stuck to you. Otherwise DVa or Zarya are nice 2nd tanks that compliment Rhein well when on Def.. Roadhog & Dva on attack imo.