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Ditto as a counter to mewtwo with ShB
Ditto will be a great glass cannon against mewtwo with shadow ball. It will die with the first ShB but a lvl 30 ditto manage to fire 2 ShB before he faint, doing more damage than anything except Ttar, and fast.
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I had thought this would be the case against Gengar but it wouldn't work because it was weak to Gengar's fast moves. Maybe with Shadow Ball Zam/Mewtwo it would work better. hmmmm.
My Lv 29 ditto (15 sta) has 79 HP, confusion Mewtwo does 15/16 damage to Mewtwo. So it looks like it could take 4 confusions, which would be ample time to charge up and send a Shadow Ball.
Mewtwo's Shadow Ball would do 120+ damage compared to Gengar's max of 117 (at lv 40).