It's not a wise idea to try and get something silly like 50k health and max resistances!

Instead, it is good to try and play supportive-pull enemies to you with fiery chain (as DK's are quite good tanks), debuff bosses with heroic slash for minor maim, use igneous shield to regain stamina and protect allies etc.

In my tank, I use bloodspawn's 2 piece with 5 tava's favor and 5 ebon armory. You may have seen a lot of builds use this-but it's really good! The main idea is that with tava's and bloodspawns (you can proc tava's with evade in the medium armor skill line) you regain a lot of ultimate which is further helped with the decisive trait on your weapon. You then use the aggressive warhorn ultimate from assault (pvp) to boost everyone's stats and damage and (if DK) regaining loads of resources.

Also, it's good to hold down block continually. You can regain stamina from ultimates, igneous shield (helping hands passive) and from the healer (hopefully templar) using spear shards. I have 100 points in shadow ward in the thief cp thing, reducing block costs by 25%