Lets do this as easily possible . lol at least I hope. Zada is on the field, its your turn. 1.) You cast ignition on zada, 2.) Zada's ability goes off 3.) Response to yourself you cast mist on zada 4.) Zada's ability goes off ... As said before, last to first. So now it goes 4,3,2,1
while remembering Zada's ability goes off when something is "cast" at specifically him, not when a spell "resolves".
Zada's ability works like heroic and prowress, go off when something is cast, not when something resolves.
So, with let's say you had Zada + 8 goblin tokens 1/1. "Igniting the Mist" would deal 20 damage flat to creatures and opponents (turn around 5-6 maybe). NICE.
If you protect your creatures, it would deal 21. You can either cast & resolve mist (or any other protection spell), then bomb, or react, keeping priority.
If you don't protect them, Zada creates copies, which are then placed on the stack on top of the original Ignition. In the case of the 1/1 tokens, they would all die from the first copy, causing the other copies to be countered for not having a legal target. End result: your board is wiped except for Zada, who received 1 damage, the rest of the board got 4.
Another nasty combat trick you can pull with Zada is
Arcbond +
Lifelink creatures. Swing in (or have a burn spell), react with
Arcbond, massive damage to everything. Due to
lifelink, you gain tons of life, offsetting the nuke you just detonated.
On the question of the
counterspell: Zada triggers (as stated by Robort) as the spell is cast. Countering the original will not stop the copies, unless you actually counter the trigger.
I've built a standard, modern & EDH deck around Zada, because I love her ability.