I finished the game using a Firestorm / Energy Shield Witch. The game gives lots of flexibility to beat Merciless difficulty BUT if we are talking about the very high end, dropped maps and beyond, most builds end up stacking as much health as possible. I say "most" because there are Totem Masters which can choose to go a little easier on the tankability if they want.

Thing is, if you stack absurds ammount of health and use your health to fuel your skills, you solved both your mana and hp problems. It's an easy solution which fits most builds you want to do, be it Lighting Arrow or Freezing Pulse. Hell, I could remake my Firestorm Witch as a Firestorm Marauder with Blood Magic and it would be an improvement.

It doesn't help if you deal too much damage you can be one-shotted by reflect damage monsters so you want to be super tanky and not too spiky.