Melmetal is absolutely bonkers in this metagame honestly, which has been stated multiple times. You only really have the Rotom's as reliable "counters" but even then, especially in Heat's case - Mel can carry coverage to dent them hard. I've been using PhysDef Rotom-H, which has over applications as well - wisping Mel and hopefully limiting its impact. But is that truly healthy for the game? Where you have to hope to limit it or use a faster mon to revenge (perferrably a special attacker, as its phys bulk can swallow most physical attacks) it? I honestly say no.