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my reasoning for including both Whipflare and Slagstorm is that I haven't found any red creatures that interact with the graveyard how I want them to. Grim Lavamancer is the only one. With that in mind I am very vulnerable to creatures and have placed both Whipflare and Slagstorm in the mainboard.
If your looking for red that interacts with the graveyard well, the card:Chandra's Phoenix is the go to card. However I also used Vexing Devil just for the turn one 4 damage. And Stromkirk Noble which is an effective sideboard card if nothing else. However Past in Flames and Burning Vengeance in all my experience never worked with Grim Lavamancer there isn't enough synergy between them, unlike with Tibalt, the Fiend-Blooded . Cards that work beautifully with Grim (that you don't already have) are Dangerous Wager , Desperate Ravings , Wild Guess , Reforge the Soul . Creatures Rummaging Goblin , Mad Prophet . These cards all feed the graveyard, if your flashing stuff back all the time then you hardly have the cards you need to exile with the lavamancer. This is just my opinion. +1