Dark Knight: Souleater (combo #3) - 300 Cure Potency
Abyssal Drain - 200 Cure Potency per enemy hit

Gunbreaker: Brutal Shell (combo #2) - 200 Cure Potency+200 Barrier Potency
Aurora - 200 Cure Potency+Regen (2 stacks)/Heart of Corundum - 900 Cure Potency

GNB does go down to 1 HP (even if full) but at least becomes immune to damage during the time and has resources to get HP back.
DRK's only way to get HP back is Abyssal Drain around enough enemies to make it count or Souleater. It makes them completely depend on a healer to bring them to full HP even if they don't know they have to. I do think they need to buff soul eater healing regardless since it is the only single target source of healing they can do. It was fine when none of the other tanks had as much healing in their arsenal, but DRK in other FF games are known to heal off damage and sacrifice health to do more damage so they shouldn't be falling behind in healing department.

Realistic Proposal to Living Dead:
Grants the effect of Living Dead. When HP is reduced to 0 while under the effect of Living Dead, instead of becoming KO'd, your status will change to Walking Dead.
Living Dead Duration: 10s
While under the effect of Walking Dead, most attacks will not lower your HP below 1. If, when the Walking Dead timer runs out, HP is no longer 1, the effect will fade. If HP is at 1 when the effect fades, you will be KO'd.
Walking Dead Duration: 10s

I also think it would be cool if they changed Living Shadow to be created with full HP and take damage instead of you for the duration instead. Could function in essence like a barrier allowing time to recover your own health.