In the Modern format of Magic: The Gathering, a few combo decks exist to take opponents by surprise and win out of nowhere. These take advantage of creative combinations of cards to create explosive effects. Ad Nauseam is one such deck, and there is also the Scapeshift deck. It makes use of the namesake sorcery spell Scapeshift, and this deck also needs Valakut, the Molten Pinnacle to deal damage and win the game.
Of course, like with any other Modern deck, Valakut has its weaknesses, and sideboard cards of any color can slow down Valakut or take it apart, piece by piece. Using these strong sideboard cards -- and using them correctly against Valakut -- can really even the odds.
Strong Sideboard Options: Hurting the Combo
Above all else, Scapeshift is a combo deck that has some elements of a control deck, since it has counter spells and doesn't necessarily win hastily on turn four. This means Scapeshift can be treated like a control deck that wins not with Celestial Collonade/Batterskull, but a combo, and a player can side in cards appropriately to deal with it.
If the player's colors allow it, side in one or two copies of Slaughter Games, a narrow but effective card from the plane of Ravnica. It can't be countered, which is vital against cards like Mana Leak and Cryptic Command, and better yet, it can "assassinate" a single card in the Scapeshift player's deck. Naming a vital card such as Scapeshift or Primeval Titan allows the caster to exile all copies of it from the hand, graveyard and library alike, and this also allows the caster to see all the cards in the Scapeshift player's deck to scout out their own sideboard options.
Thoughtseize is a powerful card against combo and control in general, being able to take a choice card from the opponent's hand to disrupt their plan and see the entire hand's worth of cards in the process. Losing two life is definitely worth zapping the Scapeshift player's Cryptic Command, Scapeshift, Primeval Titan or Summoner's Pact, just to name a few. However, Inquisition of Kozilek might not work out so well, since it can't hit the costly 4+ CMC cards that Scapeshift runs.
In white mana, Leyline of Sanctity is a great way to prevent Valakut's triggers from hitting the player directly, meaning Valakut's wrath is directed to the board instead. If Leyline's controller is running an Esper control deck, for example, they might be offering very few good targets for Valakut at all.
Also, if the player's colors allow it, they might try out one or two Counterflux, a counterspell costing UUR. It's color-heavy, but it can counter any spell and is safe from enemy countermagic. Dispel and Negate won't do any good to support Scapeshift or Primeval Titan against Counterflux.
Other Sideboard Options
Many other sideboard options can be brought to bear against Scapeshift, and some of them are generic, common anti-control cards. Blood Moon is a strong option, and in fact it may be among the most common sideboard cards in all of Modern. Shutting down fancy lands is vital in a format that has fetchlands, man-lands, utility lands and Valakut, and it works against everything from mono-green Tron to greedy decks like Death's Shadow to Esper Control.
This card won't entirely demolish Scapeshift, especially since it has cards like Sakura-Tribe Elder and Search for Tomorrow, and it might side in Repeal to bounce Blood Moon at some point. Still, this card can slow down Scapeshift somewhat and force them to wait until they can cast Repeal on Blood Moon and use their combo. That can buy Blood Moon's player a few extra turns.
Dispel and Negate are also fine options against Scapeshift, especially if the player is expecting their opponent to modify their deck to make it act even more like a control deck for games two and three. Dispel and Negate can hit Cryptic Command, Mana Leak, Remand, Summoner's Pact and Repeal, and Negate also hits Scapeshift.
In white mana, Aven Mindcensor can greatly interfere with the effects of Scapeshift, Summoner's Pact, fetchlands, Sakura-Tribe Elder and Search for Tomorrow. However, it is vulnerable to Lightning Bolt and Repeal. Its caster should be ready to either protect it, use multiples or use it at the last possible moment to minimize the odds of the Mindcensor getting countered or eating a removal spell. Shadow of Doubt is also a fine one-off effect to prevent the Scapeshift player from searching their library.