its tricky, I have been having trouble consistently trying to execute this brutality,

Your health should be below 50% in the final round and then do a combo like B+3,3,2, DB+2, 4, leg grab. for a simler one try 2,1,2 leggrab or 1,1,1, leg grab.

I would also appreciate some pointers, its not consistent.

I use back+x, x, triangle and leg grab. Be sure you have more then 50% life

I Have trouble too.

Does the last hit (as in the one that pushed them over in the edge to death) have to be the leg grab?
Or, say, if there's only a tiny sliver of health, five hit combo time into very-definitely-dead ok too?

The last one has to be the leg grab.