My opponent has a fiora peerless and i reduce its might to a negative value (through let's say frigid touch repeated). I then start a 1v1 showdown, triggering fiora. Would her negative might double or does it stay at that value?

Fiora's Might stays at that negative value — it does not double.

Why: Doubling (Rule 432) means increasing Might by an amount equal to its current value. If Fiora is at −1 Might, doubling would try to increase by −1 (a negative amount). Rule 477.3.c states that players cannot increase a numeric attribute by a negative amount — if instructed to do so, they increase it by 0 instead.

The Last Stand example in the rules covers this exact situation: a unit at −2 Might targeted by "Double a friendly unit's Might" gets +0, because −2 is a negative increase and cannot be performed. Same logic applies to Fiora's trigger.