The point about stopping early is actually quite relevant, because a deck like theirs wants to be built to aim for 4 Spirit Guides in the opener, or maybe 5, and in fact they should mulligan any 7-card hand that's distributed any other way (unless they're trying to bluff the opponent, a definite factor in a match like this). This way, they can approach the mirror match by putting out a preemptive strike where they rack up 10 Flames and intentionally stop, and still have a castable Flame in hand to respond over the top of interference...as long as they left enough fuel in the deck so that the second one can actually ripple into something lethal. If both players have a hand with two usable Flames then it's still a game of chicken and whoever shoots first loses, but if you have the double and catch your opponent on a mere single then you just win.
for the record, the ideal set-up to get that starter as often as possible is 39 SSGs, leaving you with only 21 Flames for 42 damage total. if you have the 4-3 open, then you can only do exactly 20 twice.
but anyway, given the assumption that SM is correct, anything that can gain 23 life at instant speed, preferably in chunks, will beat the optimal Flame deck.
PS:
nourishing shoal-
progenitus will beat Flames a fair amount of the time. (about 58% before mulliganing) and beats black chancellor almost every time. (about 99% before mulliganing) probably loses to leyline-memnite, although really that deck only really needs to run 6 memnites, and you might be able to deck that. but they can just rebuild to beat it by running more memnites.
PPS:
worldspine wurm is a little better because only two is enough to beat the optimal Flames build if they get a 4-3 open. (or any fewer SSGs). and it's strictly better so why not. they can try to mulligan to put as many Flames in their deck as possible but they're not guaranteed to make that work. if they get a stranded flames, you have about a 90% chance, although you then have to have a plan for them to Flames you one last time once they draw more SSGs, which is a calculation I don't really want to do. but it should happen on average.
PPPS: then again 39 SSGs starts to look dangerous in terms of actually hitting your flames. the odds are still in your favor (1 in 3 cards will be a flames, which means that you should get a double Flame hit fairly soon, and once you do you're pretty safe, but you can get unlucky and miss immediately.