Stacking debuffs

[eisprinzessin]

From my Blademaster build guide thread:

Weapon Pool Skills Blademaster

The Reduced Target’s DA for x seconds of Markovian’s Advantage and Skorpion Sting do not stack. […] Skorpion Sting will be my last celestial skill, and I intend to assign it to Blade Trap, which has -% DA. These two effects do stack. In addition -DA (e.g. in MP from Vulnerability) will also stack.

Regarding the special exception:

Buffs, Debuffs and Damage over Time
If you apply a % reduction to a monster’s damage by using a skill and then apply the same effect with a weapon attack, only the strongest debuff will take effect. The effects do not stack. In the case that a strong effect has a shorter duration than a weaker effect, the weaker effect will become active once the stronger effect expires. A special exception to this rule are debuff effects which directly modify specific stats or resistances, such as -% Vitality resistance. If two differently named debuffs (ex. Curse of Frailty and Devouring Swarm) both reduce a target’s specific Resistance by -%, the effect will stack as they are two unique debuff effects altering the same attributes.

If there was another skill like Vulnerability, then the -DA effects of both skills would stack, too. But if I am not mistaken, we currently do not have such a skill … or maybe as an item skill? Anyway the Game Guide explains that scenario with resitances, which is more common.