dont really like the team (no weather, no levitators/flyers, no fighting resists, also not a fan of volc since he requires RS support causing you to use donphan) but here are some ways to deal with those threats without changing too much
1. hp electric on keldeo lets you check gyarados with one DD. donphan still has ice shard and can take a hit from physical dragons. Alternately, you can replace keldeo with scarf rotom-w. If it has trick you can **** over blissey and chansey all the same
2. twave over toxic on chansey (maybe use blissey instead for dat lefties) helps with latios and keldeo. they'll kill you but you can revenge easily if they're paralyzed. i also prefer para over toxic 9/10 times.
3. CBtar over garchomp gives you your own weather and helps against opposing rain teams
if i were you though i'd keep terrakion chomp and keldeo but ditch the rest. I'd make terrakion the SR lead set, make chomp sd yache, and keep keldeo as is. then i'd slap on CBtar and the other 2 are for handling threats/weaknesses --- Lakers. Black Mamba. Steve Ca$h. "Boom boom" Pau Gasol "Train." Ryan "Dirk 2.0" Kelly. we. already. won.