Hey, you're right the Fire combo doesn't work. I was wrong in thinking I could cast an instant and then Fork it to combo with Kalamax. The only way I can Fork an instant to combo is by targeting an opponents instant. Casting a sorcery, holding priority and then Forking it does work to combo.
Can replace Fire with Gravitic Punch for the same win condition effect, but more difficult do it at instant speed needing flash.