If not, does the highest value or the latest debuff applied will count?ty.
They dont stack.
And the latest application sticks.
So be carefull when applying debuffs and using special attacks! 
Thanks Fledoble for the confirmation. I was thinking to bring bombs and axes to lessen the damage inflicted by bosses but then remembered that I’ve read somewhere that certain skills don’t stack. Then, i think i will use kailani and arrows/bombs instead.
You can still bring them to give a direct damage boost. 5xAxes still does quite a bit of damage.
Its a valid strategy for late stage bosses in Rare Quests and Events