It depends on how viable of options you want. Dragonite (now has multiscale) resists most of what Serp is going to be doing but will be crippled by glare and hp ice. Volcarona can switch in Serp but also doesn't like glare. Amoonguss 4x resists leaf storm and has stab poison moves to pick it off, without regenerator though I'm unsure how effective it'll be.