I think that this is probably a good strategy for Cruel Mode!

[1FC0]

Camp them from the edge using R.O.B. with your lasers and Gyro's! You should probably use the Gyro to knock them away if they get close and then regrab the Gyro!

I have a record of 6 KO's with R.O.B. I did it pretty long ago but this is probably how that I did it because I remember doing it this way. R.O.B. is amazing and so are his Gyro's and lasers!

 

[Mario & Sonic Guy]

What helped me get the 4 KOs was by using Kirby to inhale the Fighting Miis so that they're at a position where they can't recover. However, this was before the 1.0.4 update, so I don't know if the AI has been modified to counter this strategy in Smash 3DS. I do know that it's not very reliable in Smash Wii U.

 

[Quarium]

I killed 6 by backthrowing as peach back in the day. but I managed 8 simply by staying on ledge with Shulk and countering xD