Cass has the better tanking focus, Blackwall has the better tanking specialization. Both will protect you enouh that you can deal massive damage. I prefer Cass/Varric/Mage. Varric'scritical passive will also help you out tremendously with a specific Reaver ability.
That ability is Devour, and it will become the cornerstone of your offense. You'll want to lay down Rin of Pain, activate rage, build up armor, and barrier and drain your own HP for massive damage. Then, once you get low on health (50%-25%) use Devour, preferably after an upgraded Charing Bull. That, and Clear a Path are going to be very important in keepin your stamina up. Devour cost a lot, but its upgrade give it a 25% bonus to crit, plus Varric's 5%, plus anythin you're wearing. That, plus your focu will allow you to fight without going throuh potions like water.
You'll be trading stamina for damage, and armor, prioritize which is more important at any given time.