a move like heat wave if you can blast away the rain for a time will not only hurt the bug and grass types but change it to a burning field, otherwise this one is super specific but if you have a salamence and can mega evolve it, Cut with Aerialiate means ya got a flying type Cut attack that is field boosted and will reduce the field stage, but otherwise i recommend just setting that field on fire as early as you can however you have to do it, because once it's done things are immensely easier