To the best of our knowledge, the debuffs stack multiplicatively.
Example: You take Grimm (34% general defense reduction) and Jackal (-54% Holy defense reduction). The target’s effective defense is (1 - 0.34)*(1 - 0.54) = 30%, or a 70% defense reduction.
Remember that this goes into the damage calculation inside the exponent. The general formula says expected damage is proportional to (Attack / Defense) ^ 1.35. So a 70% reduction in effective defense basically quintuples the per-tile damage.
This multiplier is damped down by an unquantified but confirmed tempering of the damage as the ratio of attack to defense gets very high. You probably won’t see the tempering much unless you have big stacks of the color.