The red deck in the Challenger series is named after after Hazoret the Fervent, and for good reason. This card is a powerhouse that is difficult to deal with due to her indestructibility. With enough time, she can close out the game on almost any board state with her activated ability. This is where I would start, because she is likely to have the largest single impact of any change.
+3 Hazoret the Fervent
Harsh Mentor is probably the weakest card in the main deck, so as you are making improvements he is the first to go. He is slow and easily rendered ineffective by just about any creature your opponent has. Also, with the banning of some of the energy enabling cards, activated abilities are just not as common.
-2 Harsh Mentor
Once you are to the point of adding your 4th Hazoret the Fervent, you don’t want your mana curve to be too high or you’ll have a difficult time consistently casting your spells and emptying your hand for Hazoret to attack. Glorybringer is an expensive card and is not as necessary in the main deck. You can move it to the sideboard and bring it in when your opponents are playing creature-based decks or when they have a lot of removal and you just need to have more threats.
-1 Glorybringer (main deck)
+1 Glorybringer (sideboard)
When Glorybringer enters the sideboard I would have it replace Sentinel Totem, a very narrow card designed to beat Standard all-star The Scarab God. Since you are making your deck faster with these changes, you are hoping to be able to close out the game before The Scarab God can take over.
-1 Sentinel Totem (sideboard)
The next addition I would suggest is Earthshaker Khenra. This card is fast (because of haste), evasive (because of its triggered ability), and provides late-game card advantage (because of eternalize). Like Hazoret, the Khenra helps to punch through damage in the late game but also provides a reasonable threat in the early and middle turns.
+4 Earthshaker Khenra
Now let’s get rid of that last Harsh Mentor to make room for the better 2-mana creature.
-1 Harsh Mentor
Magma Spray helps you get through small, recurring creatures that can jump out of your opponent’s graveyard like their Earthshaker Khenras, Adorned Pouncer, or Sacred Cat. Since the Khenra commonly invalidates these creatures for one turn already or trades with them, you can remove one of the Magma Sprays from the main deck.
-1 Magma Spray
Soul-Scar Mage is also pretty weak, and you are upgrading the creature suite already, so you can take out most of the Mages while you’re at it.
-2 Soul-Scar Mage
You already moved Glorybringer to the sideboard in place of one Sentinel Totem, but there are still changes to be made. I would still like some ways to interact with the opponent’s graveyard. In order to do this, I would replace one Sentinel Totem with a Scavenger Grounds. This gives you sideboard compression because it also allows you to bring in an extra land when you are raising your curve by adding Glorybringer from the sideboard. In fact, this would allow you to play a second Glorybringer in the sideboard in place of the last Totem!
-2 Sentinel Totem (sideboard)
+1 Glorybringer (sideboard)
+1 Scavenger Grounds (sideboard)
Chandra’s Defeat is narrow, so I would rather play something that can be useful in more than red mirrors. Aethersphere Harvester is a great card versus aggro decks of all kinds because it has high toughness to allow it to block, it can gain life, it can fly over blockers, and it’s immune to sorcery-speed removal spells.
-2 Chandra’s Defeat (sideboard)
+2 Aethersphere Harvester (sideboard)
Whether your opponent is playing the Vehicle Rush Challenger Deck or a deck revolving around God-Pharaoh’s Gift, artifact removal can be the difference between winning and losing, and Abrade is the best in Standard at destroying artifacts while also removing small creatures (again providing sideboard compression)! I would replace the Kari Zev’s Expertises in the sideboard. These types of effect do not provide a recurring source of damage and don’t help you catch up when you’re behind. Artifact removal is too important and Kari Zev’s Expertise is too ineffective to justify its place over Abrade.
+2 Abrade (sideboard)
-2 Kari Zev’s Expertise (sideboard)
Key to the City helps you break through board stalls while also providing card selection. Both of these are provided already by other changes you have made to the deck, so you can free up those spaces as well. I would resort to Dire Fleet Daredevil to help me with card advantage while also being a threat itself. It can be brought out of the sideboard against removal-heavy decks to turn the tables back on your opponent.
-2 Key to the City (sideboard)
+2 Dire Fleet Daredevil (sideboard)
My Final Product
After all those tweaks, here’s what my final product would look like!
What changes would you make to help the deck be competitive at Friday Night Magic and Standard Showdown? Which Challenger deck to you think is the most fun right out of the box? Let me know in the comments!