No. It's never going to work
Sorry for the blunt answer but quite simply, your strategy doesn't work. Why?
First off you CANNOT SWITCH after user Giga Impact. You can't do anything. Just recharge.
>Deals damage. Provided the move hits, the user must recharge the following turn, and may not attack or switch.
So once you kill their Pokemon, and if they bring out a fire Pokemon, you're stuck for that turn allowing them to hit you.
In addition to this, this strategy relies on them bringing out a fire Pokemon,. What if they bring out a fighting Pokemon? Or a Rock Pokemon? They both hit it super effectively as well.
Also they could bring out a Pokemon carrying a fire-type move that you're unaware of. Sooo. Yeah this strategy wouldn't work.