Here is the finished version of the archetype I presented in rougher form a few weeks ago:
The changes reflect what often happens when decks mature. The bad cards that were better in context, Bomat Courier and Lupine Prototype, turned out not to have enough context, and got replaced by better cards, while the sideboard took on the cards you actually want in matchups that happen, rather than generically good ideas. Going down to only 1 Manglehorn is definitely suspicious, as is not having full sets of Magma Spray or Sweltering Suns, but like many decks this one has big problems with overboarding.
Your deck represents an engine for generating large attacks. That engine requires discard outlets, a good curve, and a critical mass of 4-power threats. If any of these falls short, your engine will fail. When sideboarding such a deck, a light touch is paramount. You can remove a few 2-drops safely, but after that, take great care. The reason we have 4 copies of Prowling Serpopard, even though control is not a large presence, is because all 4 copies are a large upgrade where you want them, whereas in other places it gets much harder to improve.
Manglehorn in particular makes several of your problems worse rather than better. He doesn’t have 4 power, he costs 3 mana, and you want to have a good target, so he can make it hard to empty your hand on time and often makes your key cards worse. Against Mardu, where it seems at its best, you are happy to have it when it hits Heart of Kiran, but otherwise it isn’t so mana efficient, and even when it works the 2/2 tends to be low impact. In matchups that are more about trading off the body is more exciting, but here your main thrust either runs them over or it doesn’t. Against Marvel, it does more harm than good for the same reason, and you are better off becoming better at killing them than trying to interfere with the Aetherworks Marvel itself.
Your sideboard plan against Mardu is to bring in Magma Spray to control the early game, then bring in the big boys:
Your plan against Marvel is to stick with your game 1 plan, tuning it slightly to better get past the creatures you are facing and taking out cards that line up poorly:
The Glorybringer is also an option here. Their B plan is to block you for a long time while they find and spin Marvel, which Key to the City is very good at stopping. Lightning Axe is not too bad since it is mana efficient, but your Fiery Tempers most offer enough creature removal while providing extra reach, and Key to the City substitutes as a way to empty your hand. You are good at forcing them to have it—their fallback plans are quite poor against you since they have no non-Ulamog answer for Gods.
The problem is Zombies.
Up until two days before the Pro Tour and the rise of the Zombie apocalypse, I was planning on playing Red/Green at the Pro Tour. I was satisfied with all of its matchups other than Zombies, especially Mardu. Unfortunately, Zombies, and in particular their black removal, present a problem. Grasp of Darkness can kill Hazoret the Fervent, Fatal Push can stop Honored Hydra, and Dark Salvation can often take out anything it wants.
You likely want something like this:
I am unsure how many Keys to the City you want, any number from 0 to 3 would seem plausible to me, with my guess is that 1 or 2 is right.
The deck is a lot of fun, and if you can get the Zombie matchup to a point you are comfortable with, the deck would be an excellent choice.