I battled this team alot and it's actually pretty good.
But as others have stated, Heatran is a pain for you and you also have nothing that likes to repeatedly switch into Water-type attacks. To fix both these problems you could try Gastrodon. Gastrodon can easily take Water attacks and it even gets a SAtk boost from it. Gastrodon can also take on most Heatran sets. A moveset of Toxic / Recover / Ice Beam / Earth Power will fit your team perfectly. When you don't have the Sun up it even provides you with a solid check against Thunderus and the likes. The problem, what to replace Gastrodon with, all members of your team have certain roles to fulfill. I think the best option is to use it over Venusaur. You have another special sweeper in the likes of Volcarona so not much is lost in this case.
Albeit not much, I hope this suggestion helps and good luck with the team.