Yeah, that combo doesn't actually work. You can smash DI out of it, and sometimes even smash DI outside of the second hit of the nair.
Now, whether or not your opponent will be prepared to smash DI out of the nair is a different story.
On other news, this is an odd match up for various reasons.
As someone said earlier, spacing in this match up is...
unusual, to say the least. I'm not sure how to elaborate on what to do because there is always a positive solution for the opponent to take. It's just a matter of...mindgames, I guess. Bait, put your opponent into bad situations, make quick, aggressive actions that you know will go unpunished, and don't fall into "bad spacing."
Edit:
You are aware you can reverse dolphin slash out of the dthrow to fsmash. Besides its more efficient to fthrow fsmash with a stutter step.
QFT. Dthrow to Fsmash doesn't always work because your opponent can Up+B before the fsmash lands (and the up+b will hit you). However, you don't -HAVE- to fsmash. You could raise your shield to block your opponent's up+b, then punish the lag. If it were me, I would do a fast falled fair >dtilt>dancing blade for optimal damage (I think that works, I forget.)
...So yeah.
Edit2: Tired johns.