Dude, use your brain... I saw them revive each other once and then just killed both at the same time. You see an enemys HP. You know what damage your characters can deal. If you couldnt win this battle, thats on you dude.
I did win the battle, I am not saying it's a challenge. What I am saying is that is bad game design. It killed me on my first attempt not out of skill or even rng. It beat me as I tried to thinking as a DM would. These enemies can revive no more then 3 times. Even legendary creatures like tiamat or baphomet only get 2-3 uses of a skill. A lvl 6 enemy casting the equivalent of true resurrection at infinitum is stupid