I highly recommend catching a Donphan close to the Circus with Sturdy (replace bewear)
Also, if Dewgong's ability isnt thick fat, change it for it
You probably want to switch bronzong too, even w/ heatproof unless you're trick room'in (even trick rooming, its probably still a bad idea). Since you're able to set rain up, try to get a Water type, maybe even another dewgong, or a Walrein w/thick fat.
Using X items on Rhyperior might do the work too, they're OP on reborn abuse its resistances to raise them even more early in the battle, then, when you're unkillable, boost your speed like 3 or 4 times, healing when necessary, and one stage of atk should destroy all of charlotte's mons