What do Vulnerable, Resistance, Advantage and Weakened do in Dragonfire?

These are the effects that change damage dealt or taken (values at base level):

• Vulnerable: the target takes +10% damage. An offensive debuff — you land it so your whole team hits harder on that target. Applied by Antares (and by Zivern in a +15% tactical-damage-taken version).
• Advantage (a buff): the target deals +15% damage. For your allies (Velar grants it to 2 of them).
• Resistance (a buff): the target takes −20% damage. A big defensive shield for your allies (Syrax at 6★+).
• Weakened: the target deals −10% damage at level 1. A debuff that softens the enemy.

Remember: Vulnerable and Weakened are two sides of the same fight — one makes them take more, the other makes them deal less — while Advantage and Resistance are their equivalents on your own side. Stack Vulnerable on a target before focusing it down.