Ruby Storm wants to chain cheap rituals and cantrips into a lethal Grapeshot or Past in Flames turn, so Damping Sphere taxes each successive spell they cast that turn, making it much harder to chain enough spells for lethal storm count. Skullcrack is mostly there as extra reach and to stop any incidental life gain, though its main value is finishing them off since this matchup is a pure race to see who kills first. Eidolon can actually punish their spell-heavy turns, but it dies too easily to their own removal-light plan mattering less than our own clock, so cut it for Damping Sphere's proactive disruption; Shard Volley's land tax is too slow when every turn counts.