Mono-Blue Terror is entirely blue, so Hydroblast and Blue Elemental Blast do nothing here. Hydroblast can target any spell or permanent but only has an effect if the target is red, and Blue Elemental Blast needs a red target outright. Their main deck has no red cards, so leave both in the sideboard. Instead, bring in Thraben Charm, which gives us an instant-speed answer. It deals damage equal to twice the number of creatures we control, enough to kill Delver of Secrets with even one creature on board. Tolarian Terror is a bigger ask: it has ward {2} and 5 toughness, so we need several creatures and spare mana. The Charm's graveyard-exile mode is also useful, because Cryptic Serpent and Tolarian Terror get cheaper for each instant and sorcery in their graveyard. Cut the slow Salt Road Packbeast to make room while keeping our aggressive artifact shell intact. Keep pressuring early so their mill-cantrip engine doesn't have time to discount their finishers.