Fiora, Peerless

Q. I am playing Master Yi. I have a Fiora Peerless alone at a battlefield. In the following scenarios, who wins combat? (assume no further items are added and they all resolve)

1. My enemy attacks with a single 8[M] unit.
2. My enemy attacks with a single 3[M] unit and responds to Fiora's trigger with Smoke Screen.
3. My enemy uses Smoke Screen first. Then they attack with a single 3[M] unit.
4. My enemy attacks with a single 3[M] unit. In response to Fiora's trigger, they play Kha'Zix Mutating Horror.

A.

1 - You win combat, because Fiora Peerless will be 10M after her trigger resolves. Master Yi, Wuju Bladesman's ability is a Passive Ability - meaning it will become active for as long as the condition is met.

2 - Your opponent wins combat, because Fiora Peerless will be 2M after her trigger resolves. Fiora Peerless's ability is a Triggered Ability, and will double her might upon resolution of the trigger. Because your opponent reacted to Fiora's Triggered Ability after it finalized on the chain, she had 1M (3+2-4) before the triggered ability resolved - making her 2M after resolution.

3 - You win combat. Smoke Screen has the text "to a minimum of 1M", the game only sees Fiora receiving -2M - and due to debuff only being applied once, we lock the decrease in might to whatever it was at the time Smoke Screen resolved (this is called "snapshotting"). When your opponent attacks, we then apply the +2M from Yi's Passive ability, and then double Fiora's might due to her Triggered Ability - making her 6M when combat damage is assigned.

4 - You win combat with 10M to 9M. Fiora Peerless' Triggered ability has the additional condition that she is one on one when attacking/defending - however, similar to KhaZix Mutating Horror, that additional requirement is part of the Trigger Condition and not the effect (as long as the condition is fulfilled, and the Triggered Ability finalizes on the chain, it will resolve and the effect applied).

Fiora Peerless only gets one chance to trigger per combat. If not all requirements are met, even if they are met again and her when I attack trigger was never fulfilled, she will not get her effect that combat.

383.4.e Attack Triggers are Triggered Abilities that trigger when a Unit gains the Attacker designation for the first time during a combat.
If the trigger condition contains other requirements besides attacking and if those requirements are not fulfilled when the unit gains the Attacker designation, it will not trigger in that combat.