[RATE MY TEAM] Mega Venusaur Team

[Black Waterfall]

@cfencl In my opinion, there are at least three things you should consider: 

1. Golbat will always lose 1/4 of its HP upon switching when Stealth Rocks is active;
   
2. Flying-types such as Golbat are immune to the Toxic Spikes, meaning that you cannot use Golbat to absorb Toxic Spikes even though it is a Poison-type as well;
   
3. Golbat will need a lot of bulk investment and Roost.
   

I will not say that it is impossible to make it work as a defogger (this thread is a little outdated but it may be helpful to you), but maybe there are other choices available that will fit better in your team. This is a Gen V Smogon article about defoggers and spinners.

You might want to check this very nice Smogon article about lead Pokemon too.