To be honest with my experience, you need to worry more about dps and less about tanks in hard modes. Because thats what will stop you from killing bosses.
4/5 bosses in hardmode are actually cake and you will get them in 2-3 tries if not one shot. I pugged hard mode and we had to do 1-2 tries because we had to tweak strats to meet enrage timers. Like for first boss letting BH dps to stay out in missile barrage phase and keep on attacking boss etc. Still it didnt feel like any sort of hard mode if you ask me.
The whole group can stay out on missile barrage did it with BH/Sorc heals