I know you can't DIRECTLY poison Steel-type Pokemon, but what about INDIRECTLY?? How about through Trick with Toxic orb or Fling (since Fling is Dark-Type). If these work, please tell me and can anyone tell me any other ways.
There is only one way to do it with one Pokemon: Kecleon.
An example of doing it is in this scene:
Lucario used Toxic!
Kecleon was badly poisoned!
Kecleon used Thunderbolt!
Kecleon is hurt by poison!
Lucario used Iron Tail!
Kecleon's Color change made it the steel type!
.....Other than a battle somewhat like this, it's impossible to poison a Steel type.
Well there is other ways too. 1. Have a steel type pokemon use toxic on gardevoir. One of its abilties makes it so that if its poisoned then the opponents also poisoned. I know this works cause i once did it on a geodude. 2. Have the opponent hold the ring target. It lets moves that don't effect it hurt the pokemon. 3. have a golduck on your team and read the following:
A wild klink appeared!
golduck used soak! Klink is now a water type!
With this strategy you can change the opponents steel type into a water type, making it able to take poison type moves. Hope this helps!