The Dk can tank and kill but with different setup. I have my dual-class certificates on p.atak +skill power , so that i can pve . As for tanking the worst part is that it does not have shield defense passive (you need +10 pve set to stay up at FOS 110 not to mention Gludio 110) for tank i change AP on defense + agation from p.atak to p.def, and that is enough with mid gear to tank FOS ,Gludio 110 ,SOS, and i also change from elmore to ferios as cloak and circlet from Authority to Grace (basically you need both type of gear if you play in a cp as tank ,defense and attack).
As mention above patak and crit damage need to be stacked but try to add as much crit rate as you can too (it can help when you dont have rage of valakas up as far for ignition at +16 enchant on time i have it missing just 4 sec from cd to cd).
At 114 i can solo TOH with +12 Limited cutter (as i said mid gear)