My main is Astrologian, but recently I've been leveling Dark Knight and it has helped to get a better understanding of how that job works. There is a reason why a Dark Knight could be considered "squishy" even when well geared and it's tied to their ability usage. A Dark Knight has plenty of sufficient cooldowns for trash pulls, but they may choose not to use them for a specific reason and that is Blood Price. Blood Price restores HP proportional to the damage sustained during its duration. If a Dark Knight had their own Hallowed Ground and they used that with Blood Price they would gain no MP. The use of defensive cooldowns can be perceived as "bad" as reducing the damage sustained means reducing MP gained. This is where experience and skill, or general knowledge, can separate a bad Dark Knight from a good one.

As for how to heal them, assuming you are leveling Scholar it may help to apply Adloquium before the tank goes to pull to give them an HP buffer. Don't forget about Eye for an Eye because if the tank isn't going to reduce the damage they sustain, you can. If you have the time to drop Shadow Flare that will slow them down. If you have Eos at your side those healing cooldowns will come in handy.