[Endoperez]

She's not immune to fire OR lightning. Incinerate deals 18+ points of AN damage, so three incinerates/turn should be enough as long as the mages have decent precision. Three hits from Lightning Bolt/turn or two hits from Thunderstrike/turn would also work. Thunderstrike would also deal fatigue damage, which lowers her defence (-1 defence/10 points of fatigue, more detail on pg 76).

Pg 5 of the manual gives details about chances of beating difficult rolls. Base penetration 10 (sor S2 caster casting Paralyze) against mag res 20 (no S) is -10 in the table, or 3% chance. For S4 caster (base penetration 14), the chance would be 6% already.