I heard that if a Lv.100 magikarp used flail in battle the opponent's Pokemon will be KOed. Is this true?
No. I have check many resourses and have found no evidence to hold your claim. you may be mistaken or you heard this because of ingame battling. At 1 HP flail has a power of 200(Pretty impressive). So if a friend told you that it could kill anything it could just be that it did 200 damage and can kill the competition. Here is the link for the move Flail. If I have been mistaken because of bad resourses please tell me because I am very sorry. So at lv. 100 it would do a lot of damage.
No, it is not true at all, unless the Magikarp is of a way higher level.
You can never tell for sure if a OHKO will happen,
Unless you use always OKHO moves. It is because a Poke'mon
could resist the move and Magikarp is pretty slow so it might
faint before you get the chance to attack.
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