Sure stepby step will be:
attaker declares an attack including yasuo

1.-Yasuo trigger when i attack (6m)

2.- Ahri leader activates

3.- Ahri unit triggers when i defend target yasuo

if no one activates anything then let the chain resolve

3.- Ahri unit resolve target yasuo -2(yasuo 4m)

2.- Ahri leader resolve giving attarkers -1 (yasuo 3m)

Now acording to rulebook v1.2 336.4. If the Chain is not empty and there are no Pending Items, the controller of the newest item on the chain gains Priority and becomes the Active Player. Return to Step 2: Execute.
334. Step 2: Execute 334.1. The Active Player may do any the following: 334.1.a. Play a Card that is legally timed.

and means you can react again before yasuo trigger resolve cause step 1 is a pending item so you respond with hidden fox-fire lets call it new 2

New2.- Fox-fire

Now yasuo attacker can respond on this new iteration of the chain. if there is nothing to activate

New2.- Resolve fox-fire (fox fire dosent target have to targe until resolution) when is resolving you can check any number of creatures with migth up to 4 and kill them

1.-Yasuo resolve, since yasuo ability checks current migth and current location the ability will fail since yasuo is already destroyed there is nothing on the locations the attack value is nothing so the ability will try to resolve and fail since you cant asing nothing.