Err, why is Dragon barred from submissions when Bug, Steel and Poison are far worse matchups? I mean, Dragon is actually a neutral matchup for the type, I don't think there would be any problems on allowing it (Yeah you could have stuff like Kyurem-Black and Latios as classic checks for it, but I don't think that's much of an issue though)
Steel: Heatran is instant counter for the set you mentioned, needless to say.
Bug: Venusaur somewhat struggles vs Volcarona thanks to its ability to set Quiver Dance and Pinsir destroys it. (Bug teams are a big threat most of the time)
Poison: This can be somewhat difficult for Grass with the lack of proper pokemon and sets. Pretty much nearly everything resists Giga Drain + Sludge Bomb combo, which's kinda redundant having it allowed for this challenge as well.
I'll explain how i choose the types : Consider there's 2 kind of pokemons here : Defensive (Walls, Support and Stallers) and Offensive (Choiced, Sweepers and Setup Sweepers). For Offensive mons i allow the types which are
Weak (which have a bad matchup) against the pokemon (not the type), and for defensive i choose the types which
struggle (so which has a neutral/bad matchup) to pass the pokemon in 1v1. In this case, Mega-Venusaur is more a Wall than a Sweeper, even if it's offensive set.
• Steel : So, if a type has a instant counter i can't allow it ? Only few Steel pokemons can won the 1v1 vs M-Venu (Magnezone Specs, Jirachi, Metagross AV and Heatran) so i think i can allow this.
• Bug : My bad, i remove it.
• Poison : Same thing as steel, the difference is many Poison pokemons wall M-Venu, but they can't all won the 1v1 (only Gengar, Drascore SD, Dragalge and Crobat/Golbat).
• Dragon : Like almost played pokemons won the 1v1 (only garchomp can't), so it's not allowed.
Also, i'm allowing types like Poison for this one (which has a good matchup) to make the project and submiting a bit easier. But if i'm wrong don't doubt to tell me (in that case PM'ing is better because of flooding).