To beat Griselbrand, I find that Vexing Devil is great at helping at least to get opponents down to 7 before they can activate his ability. I'll grant that I have no experience with systematic testing but given the recent growth of Reanimator and Sneak Show, I wouldn't run fewer than 3 and have had good experiences. I would rather cut Lavamancer to fit them in than Keldon Marauders, which guarantees 2 damage once it resolves, provides 3 more potentially, and can play the role of chump blocker for opposing fatties like Goyf and Knight of the Reliquary. I also think that Hellspark Elemental is important to keep in for dodging counterspells and discard, in addition to providing reach similar to what Lavamancer can offer. Grim Lavamancer, however, tends to hit only for 1 each turn until the lategame, likely to be blocked and unlikely to survive. In the current metagame, he likely either faces a removal spell or you're playing a combo deck without removal, in which case he's a dead card anyway.
Though he wins our hardest matchup, Vexing Devil always comes out against anyone with Swords to Plowshares, usually for Sulfuric Vortex and some combination of Sulfuric Vortex, Smash to Smithereens, and Red Elemental Blast (depending on which deck exactly I'm against).
Also, remember that Pyroblast can more easily be Misdirected, which we can easily fix with the switch to REB. While Pyroblast can hit any target just to get threshold in a pinch, we can afford to miss out on that more than we can afford to have it hit with a Misdirection.
I was wrong about the Misdirection/Red Elemental Blast thing; ignore it.
