Mono-Red Sligh's aggression comes from small, fragile creatures — Jackal Pup, Mogg Fanatic, Grim Lavamancer — all of which die to Pyroclasm's 2 damage, and even Ball Lightning is gone at end of turn regardless. Hydroblast can counter or destroy any of their red spells or permanents, giving you an answer to a resolved Cursed Scroll, Seal of Fire, or a burn spell aimed at your face in a race. Tsabo's Web is dead weight since their manabase runs no vulnerable tapped utility lands, and City of Traitors is a marginal cut in a matchup where you want your mana reliably available rather than sacrificed. Keep Defense Grid in, since it taxes their cheap burn on your turn and buys crucial time to assemble the combo.