Question about damage debuffing and res

Like let's say you had 4 time defenders with junture stacked on a group of enemies. Is the debuff additive to the cap of -90% damage? And then if you have 70% or 75% or whatever the cap is for defenders resistance to the damage type they are using, does that turn a 100 damage attack to 2.5 damage?, with a 5'ish% chance to land of course lol?

What damage debuffing does is lower their damage as they are attacking, just as a damage buff increases your dmage before attacking. So say they deal 100 damage and they are debuffed down to 10% of that because they have a 90% dam debuff. The 100 point damage attack now does 10 points. If you have 75% resistance, it applies after the debuff, and you would take 25% of the 10 points, so you would take 2.5 points. This allows a psuedo "above res cap" tactic.