Sideboard Theory: (WIP)

> R/B Midrange:

Their Problem Cards:

Misery's Shadow - Can get big enough to kill or trade with our creatures, stops Old-Growth Troll death triggers.
Bonecrusher Giant - Stomps our mana dorks when we're on the draw, can trade with most of our creatures.
Graveyard Trespasser - Can trade with most of our creatures when it flips.
Sheoldred, the Apocalypse - Can trade with all of our creatures, deathtouch makes it almost impossible to do better than 1 for 1.
Extinction Event - A lot of our creatures are three mana, making this a big blowout if they land it.

No changes.
Just keep playing creatures until they run out of removal and be aware of the problem cards, we can make many 2 for 1s and grind them out.

> Nykthos Ramp:

Their Problem Cards:

Cavalier of Thorns - A 5/6 will block all of our non-buffed creatures.
Karn, the Great Creator - Enables the combo, can fetch silver bullets like The Mightstone and Weakstone and Cityscape Leveler.

In:
3x Inscription of Abundance - A +2/+2 boost will give us enough to kill a Cavalier, and the instant-speed fight has some uses when attacking.
2x Vivien, Arkbow Ranger - Much the same as Inscription, bigger creatures and fighting plansewalkers.
2x Ghalta, Primal Hunger - Become the beatdown, take out plansewalkers through a Cavalier.

Out:
4x Werewolf Pack Leader - Almost nothing in their deck is smaller than a 4/4.
2x Lovestruck Beast - Stalling with a big blocker doesn't help when they can combo, shaved two.

> Gruul Boat

Their Problem Cards:
Bonecrusher Giant - A 4/3 trades with a lot and can Stomp our dorks, tons of value from one card.
Skysovereign, Consul Flagship - A flying 6/5 is difficult to remove and hard to race, will most likely need to be fought or forced to block.
The Akroan War - Steals our biggest threat and hits us with it.

In:
2x Inscription of Abundance - A +2/+2 boost will give us enough to kill most things, and the instant-speed fight has some uses when attacking.

Out:
2x Quirion Beastcaller - Going straight to the 3 drops is probably better here, will come down to a race to establish board.

> G/W Angels

Their Problem Cards:

Almost all their creatures have flying and a lot have vigilance so blocking/attacking becomes tricky.
Skyclave Apparition - Removes all creatures
Bishop of Wings - A 1/4 for WW is very strong, especially when it enables powerful synergies, we need to get out the gates early and force them to chump with it.

In:

Out:

> Lotus Combo

Their Problem Cards:

Emergent Ultimatum - Tutors for their combo
Languish - Kills almost all of our creatures.
Ritual of Soot - Kills almost all of our creatures

In:

Out:

No creatures and no removal, a classic combo vs aggro race.

> U/W Control

Their Problem Cards:

Fateful Absence - Efficient removal.
Supreme Verdict - Kills all of our creatures
Farewell - Kills all of our creatures
Aether Gust - Efficient removal.

In:

Out:

>Mono-White Humans

Their Problem Cards:

Brutal Cathar - Repeatable removal.
Skyclave Apparition - Removes all creatures
Declaration in Stone - Efficient removal.
Destroy Evil - Efficient removal.

In:

Out:

>Abzan Greasefang

Their Problem Cards:

Greasefang, Okiba Boss - Enables their combo, kill on sight.
Abrupt Decay - Efficient removal.

In:

Out:

>Izzet Phoenix:

Their Problem Cards:
Thing in the Ice - Can force us to return all our creatures to our hand if it flips.

In:

Out:

>Five-Colour Fires:

Their Problem Cards:
Fires of Invention - The engine of the deck.

In:

Out: