I would lie if I said, it's easier on HGSS...I mean, of course I was just a mere boy, when I fought against her Miltank on the original one and really had no strategy. Thinkin' back on it now...There are ways to stop Rollout. Like catching a Drowsee and using Disable once it start doing it.
If your level close to her Miltank, then the rollout won't be instance KO strong, till it reach it's third or fourth roll. Or also, Hypnosis with a Gastly could be helpful, sleeping will cause a reset to Rollout, once it awake.
I actually found her harder on HGSS. Scrappy was just straight BS. It's get a stab boost with Stomp and is pretty much faster than ANYTHING in your party. Flinching EVERYTHING!
And using a Drowsee or a Gastly is actually pointless...God, it was so FUXIN' cheap.
-__- I hate Whitney on bother versions HG/SS and G/S/C