Slow poison

[A-Man]

If you edit the defend frame of every character that will be able to not get poisoned, yes. (Adding a special body that will make the poison object disappear)
But that's not a very practical option.

No, that is a wrong way. If he will follow it then he have to even edit the newly created or downloaded characters. If he is making a mod then it's ok but if he is making a new character who will have this attack and then we will download it then we also have to edit all the characters. That would be very annoying and difficult.

(Sorry if I had made any grammatical mistakes)

The special bdy is the only method (AFAIK) to make it bug-free, and I am sure the OP understands that as he is the one who asked how.