Aren't dummys in the training mode supposed to be unkillable?
Well I was practicing my ass in the training mode with 
and I killed some (unkillable) dummys.
I was just spamming W and R until I got a penta.Such skill.
2 Comments
it works with grevious wounds , pick any champion and buy Last Whisper but the one which gives u Grevious wounds and enjoy getting Pentakills . and since katarina's R automatically put enemies inside it on grevious wounds so they will die ( The target dummies ). and btw Vandiril made a video about this thing a week ago.
I'll add that the 
Morellonomicon also has the same effect.
Dealing magic damage to an enemy champion below 35% health inflicts Grievous Wounds to them for 8 seconds.