she’ll take no damage as long as potency isn’t above 100%. So torch for example increases his potency by 10% per smoulder, meaning he can increase the damage past 100%
Specifically the way this works (and similar resistances like Immortal Abom’s poison resistance) is that they reduce the potency of incoming effects (of the relevant type) by 100%. So if nothing boosts the potency, it’s 100-100=0% potency. If, for example, you’re going against an Enhanced Incinerate node that gives +40% incinerate damage, then you end up with 100+40-100=40% net potency. You take less damage but not zero.
Is this your first time with "resistance/reduced potency/takes -x% damage" ability in the description?
You should ask Onslaught why he needs 150% incinerate potency reduction