Ok, I finally see what you're saying. I will give you the calculations, but as all Pokemon can have a different max HP stat, I'm NOT doing one for each of the 649 Pokemon. Here are the calculations:
All passive damage moves do 1/16 of the opponent's max HP in indirect damage. All partial-trap moves (Bind, Clamp, Fire Spin, Magma Storm, Sand Tomb, Whirlpool, Wrap) do 1/16 damage on every turn active, unless the user holds a Binding Band, which would make the last turn of the damage does 1/8 of the opponent's max HP. Toxic does 1/16 damage of the opponent's max HP, plus 1/16 of damage every turn. Using these calculations, the opponent would be Knocked Out in six turns, as the 1st turn is 1/16, next is 2/16, and so on. Normal Poisoning does 1/16 every turn. Hail and sandstorm do 1/16 max HP damage, except to Rock-, Ground-, Steel-, and Ice- types respectively. So, in the first turn, the maximum damage would be 1/16 of the foes max HP. This means that it would take you 13 turns to KO the opponent. Using these calculations, we can use Blissey to figure out the highest damage possible. Let's skip that :P I did it in my first answer. Here is the total damage after each one has activated (the first turn each damages in unison) : 315 HP damage to a Blissey with maximum HP (714). Here's the stuff you need to know:
It takes 13 turns to do 100% of the opponent's max HP in the situation you described (I send Onix out first. He's holding a Smooth Rock. Uses Sandstorm, switches out. Then, I put Flareon. Uses Fire Spin, then Toxic.)
The first turn they all damage together, the opponent will have 9/16 of theirmax HP left.
The Pokemon with the highest HP stat (Blissey) would lose 315 HP when they damage in unison, which means 315 is the highest damage possible.
My calculation I did on a seperate sheet of paper:
TURN DAMAGE MOVE UNISON 16TH DAMAGE
1st Turn- Sandstorm 4/16
3rd Turn- Fire Spin 2/16
4th Turn- Toxic 1/16
1st unison turn---4/16, 2/16, 1/16 = 7/16
2nd unison turn--5/16, 3/16, 3/16 = 11/16
3rd unison turn--6/16, 4/16, 6/16 = 16/16 (100%)
Do NOT hide this if wrong, as this took me 15 minutes to do.