Activate your mystery before the lower initiative ninja in the 4th moveslot use standard attack. Your mystery will go off right after clones attack, before the next round. Any later and you will not be able to use mystery for that round.

For example, if you have lower initiative, and you put kisame in the 4th slot. You will not be able to launch any mystery this round with the chakra he drains from his standard attack; it will be too late. If you have higher initiative in this case, then you can squeeze out a last second mystery.

You will need to activate mystery before any slot 4 ninja take action if your computer suffers from frame lag.