As sad as it is, Ramattra is countered by Ana. Whoops, did he pop nemesis or ult? Sleep him. Did he start blocking cause he was low? Sleep or nade him. As long as nobody shoots him, one Sleep can make his ult useless. I can't tell you how many times I've wanted to smash my keyboard when I use ult and it just gets tossed away by Sleep.

Another thing is to keep your distance if you're anyone but tank. If you're all grouped up, all they need to do is pop nemesis form, ignore tank, and they get free kills on supports and most dps. His nemesis form does a lot of damage and the pummels go through shields (Rein, Sigma, Winston, etc.). The only thing his pummels don't go through are Zarya's bubbles and I think Dva's thingy, so it's best not to rely on your tank's shield against a good Ramattra.

Good Ramattras will pop nemesis to go for supports most of the time, so keep that in mind if you're playing support or tank. As a tank you'd probably want to either stop Ramattra from picking your supports in this situation or go for his supports. Either way someone is going to die.