What happens to Fiora Peerless bonus Might if my enemy ambushes in a unit?

Yes, Fiora's Might still doubles.

Fiora, Peerless's ability checks the "one-on-one" condition only at the exact moment she is designated as the attacker or defender — not when the trigger resolves. When the opponent plays an Ambush unit as a Reaction, the trigger is already on the chain. Since her ability uses "when" (a triggered ability) rather than "while" (a passive ability), it does not re-check the condition at resolution. The doubled Might is locked in regardless of the new unit entering the battlefield. (FAQ #11178, #1231, Rule 383.4.e–f)