I think a 6v6 with no level cap could work, but it may be unfair for people who got more time to level up pokemon.

We can also do a 6v6 lvl50's, but you would have to stop around lvl50, or it won't work.

We can also do a 3v3 auto lvl50. This may be the most fair of them all, but I see 3v3 being fast and less fun than 6v6.

Clauses would be:
Sleep Clause
You cannot put two of your opponent's Pokemon to sleep at the same time. (Rest not applicable)

Frozen Clause
You cannot have two of your opponent's Pokemon frozen at the same time.

One Hit KO Clause
One hit ko moves such as Horn Drill or Sheer Cold are not to be used.

Item Clause
4 or more Pokemon cannot hold the same item in your party (the max is 3) with the exception of the choice items and focus sashes(one each per team) One choice scarf, one choice band ect.
---The item Soul Dew is banned.---

Selfdestruct Clause
The moves Selfdestruct, Explosion, Perish Song, Destiny Bond, or any other such move may not be used if both players only have 1 remaining Pokemon. If a player uses one of those moves under this condition, they automatically lose.

Banned List:

-Mewtwo
-Mew
-Ho-oh
-Lugia
-Kyogre
-Groudon
-Rayquaza
-Deoxys (all forms)
-Dialga
-Palkia
-Giratina (both forms)
-Shaymin (sky form)
-Darkrai
-Arceus
-Manaphy
-Wobbuffet
-Zekrom
-Reshiram
-Kyurem