Mono-Blue Faeries plays no red permanents for Hydroblast or artifact/enchantment spells worth countering with Annul, so this is another matchup where the sideboard offers minimal specific tools. Arms of Hadar is a slight upgrade over Suffocating Fumes against their army of small Faerie tokens and 1/1s like Faerie Miscreant and Snaremaster Sprite, giving -2/-2 to wipe their board more reliably. Keep the core counterspell suite of Counterspell and Spell Pierce intact since this mirror-like matchup is decided by protecting your threats and disrupting theirs. Prioritize Ninja of the Deep Hours and Okiba-Gang Shinobi to punish their small ground-stalling creatures via ninjutsu.