DRK's defensive abilities lies in the available gear as well as our subjob choice. If you need to survive a long time as a DRK, either throw on a ton of +HP gear and use Dread spikes with your AF3+2 body on, or throw on a bunch of -PDT and -MDT gear.

Urteil would know more on the subject than me since he crafted his DRK to adapt to virtually every situation ever.