As for MTing DRK needs some help with mitigation and again it doesn't need much.

Living Dead: Remove the death effect... really that is just a stupid mechanic; or if you must keep it then change it to 50% of total health and lower the CD to 3 min

Souleater: increase healing to 150-200% and add "Dark Arts Effect: add a magic damage shield for 100% of health restored (including overheal, so that you will still get the shield if used at full) OR add 150-200% magic dmg shield of the damage delt while in grit stance and remove the DA requirements for the shield

Dark Arts:Remove evasion buff with DA changed to increase parry rate to 60-80% (alla bulwark)

IMO with these changes DRK will become a Strong Magic dmg tank that you would rather have over a PLD for fights like A4 where almost all if not all of the dmg is magic based and the overall playstyle of dark would not change.