Sentret's post-game, I believe.
And you have a hard time figuring out HM slaves in Platinum? 0_o I know I'm just walking into a set-up here, but... Bibarel. Best HM slave since Linoone. =P You can literally give him four HM moves, if you wish, including, I believe, Waterfall. Definitely Surf.
Bibarel/Tropius is THE best HM slave set, but it takes a while to get either of them, and at the time the Bulbapedia HM Slave article didn't list Bibarel for some unknown reason. Ended up using a Geodude for Rock Smash and something else (can't remember what) for Cut, before I found a route where I could catch wild Bibarel. (Because there was no way I was going to train up a Bidoof, I absolutely
loathe their cry.)
