Hmmm, given noncreature spells don't even make up a quarter of your deck I doubt you are getting much advantage using Monastery Swiftspear's prowess ability. I'd recommend cutting it for Goblin Guide or Fanatical Firebrand.
Impact Tremors is only really good if you're spamming tokens or using something like Norin the Wary. Once in top-deck mode you'll only be playing one creature for turn if you're not drawing lands or non-creature spells. I would run Curse of the Pierced Heart over it, because it's much more consistent and deals damage practically the turn it comes into play.
Grim Lavamancer is only really good if you find it easy filling up your graveyard. Most burn decks run a bunch of burn spells and fetch-lands to fill up the yard so that they always have fuel to burn. Yours lacks this and you definitely don't want to exile a flame jab if you can help it. There are some discard effects, but I don't think it would sustain a lavamancer for enough turns that you'd want it to. Maybe consider Flameblade Adept, but more discard effects would probably be preferable for it too.
Scorched Rusalka is kind of dinky. I understand sacking the Marauders and Elemental for value does seem good, but for 1 damage doesn't feel like that good of a deal. I think Hellspark Elemental would end up dealing far more damage for you.
I think if you were to running Goblin Guide and Fanatical Firebrand with your Goblin Fireslinger and maybe cut out two mountains for 2 Mutavaults you could probably justify running Goblin Grenade which would probably help give your deck more of an edge.
October 2, 2018 8:59 p.m.
Some of the mechanics of this deck different than typical Burn, Pyromancer, Storm, Goblin, or Hollow One decks. In 8-Flame you want to increase the number of damage triggers on a specific turn while you benefit from +2 damage and double damage modifiers.
I'd like to clarify the use of several cards for StopShot and anyone else who's been wondering about card choices.
Regarding Monastery Swiftspear and prowess triggers, there are actually 14 noncreature spells and an additional 20 lands that can be used as "retrace" targets of the noncreature spell Flame Jab. That means more than half the cards in the deck can trigger prowess!
Unfortunately Curse of the Pierced Heart must deal damage during the opponent’s turn when it should not or cannot be modified by either Quest for Pure Flame or The Flame of Keld, respectively. If Curse of the Pierced Heart trigger on it’s owner’s turn after the draw step, it could be a staple in this deck.
The graveyard card sources for Grim Lavamancer include the 8 self-sacrificing creatures Spark Elemental and Keldon Marauders as well as the 8 Flame enchantments, the 4 Flame Jabs (only one Flame Jab needs to remain in the graveyard for “Retrace”) and some of the 20 "Retraced" Mountain cards. The Scorched Rusalkas can sacrifice your other creatures on demand. Most of the deck and go to the graveyard and fuel the two Grim Lavamancers!
Scorched Rusalka has more good uses. It can sacrifice itself and other creatures you control after combat the same turn that Quest for Pure Flame and The Flame of Keld are increasing damage. (You can achieve as much as 6 damage for each creature sacrificed with just one Quest for Pure Flame and one The Flame of Keld modifying the damage!) Scorched Rusalka's ability can also be done at instant speed in response to removal. Scorched Rusalka helps fuel Grim Lavamancer.
Having cleared up reasons for card choices, I do appreciate your advice. Goblin Guide, Fanatical Firebrand, and Goblin Grenade are attractive options. I may try them later in a goblin variant of the 8-Flame deck!