I've heard that if a Pokemon was all types,it would only be weak to the rock type.
is it true ?
I Found it!
>If there were a Pokémon of all 17 types, it would be weak to only Rock-type moves, due to the fact that there are more Pokémon types weak to it than Pokémon types that resist it, and that no types are immune to it.
Source(s) Bulbapedia Click here and scroll down to Trivia
Hope I helped //Jofly