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to get toxic, you need 4500 coins and some cash. then, you need a high leveled pokemon, and 5 goletts/golurks(any level)
teach one of the goletts toxic and keep your highest level pokemon in 6th slot. keep golett with toxic on first slot and fight myst. trainer. use toxic on first turn, and then use megahorn(don't worry about damage done). after that, if your golett faints, switch to another. when 9 turns pass including your first one, the turn at which you used toxic, switch to your main high leveled pokemon and use megahorn. regardless of damage done, the mew will faint. you will gain lots of exp.

as complan boy said u can get it in dep.store , it is not strict to teach golurk or gollet a toxic u could have ur own choice .

It is not strict to teach toxic to golett/golurk but its its one of the best choices to put it on along with fly. Other than those pokes you can also put it on other ghost types pokemon. I would not suggest putting it on a non ghost type because they will be easily fainted again mew.