Time to choose an adventure. This scenario has a ton of different plays, and no clear right answer, so let’s get down to it.
• You are at 20 life. You can definitely take a hit or two, so you don’t need to play super defensively.
• You can cast any spell in your hand, but may have to sacrifice Scions for some of them.
• Attacking with Forerunner of Slaughter can be tacked on to any play, as trading with either untapped creature is good.
Cast Brood Monitor or Brood Butcher (I’m combining these into one line). Casting either creature is certainly reasonable, as the 3/3s block well, and leave you up a bunch of Scions. You do have to sacrifice a Scion if you cast Monitor, but you still end up with four in play. Regardless of which you cast, it isn’t worth sacrificing a Scion to use Forerunner of Slaughter to give your new 3/3 haste. You could also cast Brood Butcher by sacrificing two Scions, which would let you leave BG up and activate the Butcher. Likewise, you can sacrifice an extra Scion to leave Swamp untapped when you cast Monitor, leaving Complete Disregard up.
This is ultimately a defensive play, as the Scions aren’t attacking anytime soon, and the 3/3 is doing more on defense than offense. If you are planning on making this play, you should attack with Forerunner first.
Cast Complete Disregard. This play is too mana inefficient. Do not do this.
Cast Demon’s Grasp. You have to sacrifice a Scion, but you do get rid of one of the two big threats. If you cast Demon’s Grasp, I’d lean toward killing the Blight Herder, since you have good ways to block the Barrage Tyrant and your life is high enough that the Tyrant’s ability isn’t that threatening. I would not attack with Forerunner in this case, as you want to keep it back to block Tyrant after picking off the Herder.
Cast Mire’s Malice without awaken. Sacrificing two Scions to make a land into a 3/3 is bad value and needlessly risky, so I’m just looking at the unkicked version of Malice. This is an appealing line because it puts the opponent to zero cards in hand, and they could easily have another expensive spell they were planning on casting the next turn. I wouldn’t attack if I went with this line, because you need Forerunner back to block the Tyrant. Additionally, you can double-block the Ruination Guide with Scions if that attacks. You do take 5 from Blight Herder, but your 20 life points help make that less painful.
I’d go with Option #4, the Mire’s Malice play. The board is still close to even without you adding to it, and knocking the last two cards out of the opponent’s hand has to make you a huge favorite to win this game. The other plays are mostly defensive ones, but you are in good enough shape already that I’m willing to pay some life to give the opponent the fewest outs possible. If you were racing, I’d be more inclined to play creatures, but that’s not what these options let you do.
I’d plan on trading off as many creatures as possible if the opponent attacks, though there’s a chance they just send Herder. They may also throw Guide at your Forerunner, in which case I’d chump block Barrage Tyrant with a Scion. You then get to untap and take control of the game with the four great spells in your hand.
Cast Mire’s Malice without awaken. Block as many creatures as possible on the following turn. Win the game.
There are a lot of options here, so feel free to explain why you like going down a different path.