Belcher is a fast combo deck built around Goblin Charbelcher and a suite of blue counterspells like Counterspell and Force of Negation, so a discard spell that only sees one card is far too slow and unreliable to strip their redundant combo pieces. Mystical Dispute is cheap against their many blue spells and can counter their protective Counterspell, Disrupting Shoal, or a key enabler like Whir of Invention. Teferi, Time Raveler shuts off their instant-speed interaction like Sink into Stupor and Jwari Disruption during our turn, letting our own spells resolve more safely, and its -3 can bounce a problematic permanent. Fatal Push has few good targets since Belcher runs almost no creatures, so it comes out; keep Force of Negation and Spell Pierce in the maindeck as your primary counter magic against their key spells.