I'm working on a TMNT4 speedrun on hard mode and I made boss killing movies, including some of Slash. The first one is done without shoulder charging, and the second one is done with shoulder charging.
These aren't perfect by TAS standards since I played in real time, but they should give a pretty good idea of how the Slash battle works on Hard (I think these are faster than your current run).
http://dehacked.2y.net/microstorage.php/info/766/slashbattle1.smv
http://dehacked.2y.net/microstorage.php/info/767/slashbattle2.smv
About the first Slash battle, I haven't tested using a special against Slash's roll attack (which will prevent him from traveling to the other side of the screen), then hitting him on the way down. Hitting Slash multiple times in one shoulder charge might be possible if you're overlapping him, I never tested.
All_Bogs: I have done some testing against the Krang battle and it is one of the few battles in the game that is random to my knowledge. There doesn't seem to be any pattern to what height he will appear at.