Challenge - Reducing power is neither green nor red. I'd reverse the effect to "Choose target creature an opponent controls. When that creature dies this turn, creatures you control get +2/+0 until end of turn. Target creature you control fights that creature."
Chameleon Skink - Tapping creatures is white or blue.
Champion of Chaos / Favor - "Chaotic" is a race? Odd. Favor's reminder text is awkward because unlike Hearthstone, Magic's rules don't keep track of which creature 'killed' which. "Whenever a creature dealt damage by this creature this turn dies this turn" or something like that.
Chaos Spearman - Way too good of a blocker to be a common.
Chaos Warrior - Not sure why the blue in the ability is there.
Destructive Duo - "As an additional cost to cast ~, sacrifice a land"
Dwarf Miner - Either "Target creature with power 2 or less can't be blocked this turn" and lose flash, or just become a blue card.
Felix Jaegar - That affinity ability is really specific just to make him cost RW. I'd drop it, especially considering his enter-the-battlefield ability means you want to play him before the other legendary.
Furious Warrior Priest - "1, T, Discard a card:". Unless you want players to be able to activate it while having no cards in hand.
Gift of Chaos - Having two different kinds of counters on a creature is a giant memory issue. If I have to ask a player whether a creature is a 2/3 with mutationx2 or a 3/4 with mutationx1, and they might not even remember, that's a problem.
Goblin Shaman - "Land card". If it's not on the battlefield, it's a "card".
Gorbad Ironclaw is incredibly unfun to play against. He basically ensures your opponent will never cast anything that costs 6/7 + mana, and if you get ahead on board you just win.
Herald of the Horned Rat - I don't like the internal conflict in this card - I want to have 3 black mana to cast it as soon as possible, but it's often pretty useless without green mana to activate its ability.
High General - not flashy enough to be mythic, and that second ability is way too specific / parasitic (only working with certain cards).
High Lord Kroak - His last activated ability isn't green or blue at all, it's red.
High Lord - Should be white. Vigilance and first strike aren't blue.
Horrors of Tzeentch - Blue/black colors on this is strange. Dealing damage is primarily red. If you want it to be monoblack, maybe: "Whenever ~ attacks, it deals 1 damage to you and 1 damage to target creature or player" or "2B, T: ~ deals 1 damage to target creature or player. You gain 1 life" or something.
Idol of Gork - Artifacts should only be colored if you have a strong reason for doing so, such as phyrexian mana or enchantment artifacts. This just feels like an enchantment.
"Imperial Rifleman deals 2 damage...". All damage has to have a source, otherwise "Protection From" doesn't work. This card is repeatable removal that seems too centralizing.
Konrad - Are you aware that because it deals damage twice in that first combat phase, it gives you two extra combat phases? Because that doesn't seem intentional.
Master Moulder - "Tap an untapped Warpstone you control". Seems a bit too wordy/complex for a mythic.
Night Goblin Fanatic - More black/red than black/green.
Night Goblin - "Whenever" for things that usually trigger more than once such as attacking, "When" for things such as enter-the-battlefield abilities. I'd also make this green, because black 2/2s for 1B usually get minimal to no upside.
Night Runner - Like I said for Chaos Spearman, deathtouch and first strike on the same card should not show up at common. ([[Pestilent Kathari]] is an exception but it's...kind of overcosted.)
Sets aren't supposed to have +1/+1 counters and -1/-1 counters in them, because the interaction between the two is weird (If a creature has +1/+1 counters and -1/-1 counters on it, they 'cancel each other out' and both get removed at a 1:1 ratio as a state-based action). It's not intuitive that if you put a -1/-1 counter on a monstrous creature that it just loses a +1/+1 counter and that your Priest of the Plague doesn't get buffed any more.
Raging Bloodletter is too powerful/swingy. [[Markov Patrician]] changing life totals by a total of 6 every time it connects is actually considered a red flag at common.
Haste is tertiary in green, meaning it shows up a little bit but isn't very common.
Screamers - Probably too dominant/powerful.
Screaming Doombell probably costs too much.
Skaven Slaves - "Another target Skaven creature" to prevent it from blocking, then tapping to buff itself and becoming a 2/2 blocker.
Skink Priest - Sacrificing creatures to draw cards is more of a black ability.
Slann Mage-Priest - Last ability is red.
Tyrannical Stonehorn - Doesn't seem very mythic.
Warlock Engineer is way too complex for mythic. Think [[Worst Fears]] or [[Platinum Emperion]]. Mythics are about "wow" factor, not complexity.
Warplock Jezzails - Makes no sense as a black/green, monoblack, or monogreen card.
Warpstone Typhoon is pretty weak. See [[Caged Sun]], [[Mana Reflection]], or [[Mirari's Wake]].
Balthasar - His RW ability doesn't make sense for his colors, producing any color of mana repeatedly is mainly green. His RW ability can be activated again using the mana it produces. Thus, infinite mana. And with his UR ability, he can draw your entire deck with that infinite mana.
Black Orc Marauders - A 5/5 for 5 -> 4/4 is probably too good for common.
Bright Wizard - Repeatable removal is bad for limited. This would dominate the format.
Brutal Ambush - Making a bunch of haste tokens isn't green.