I think this series is going to boil down to how good the Bergeron line can be. Ottawa's strategy this year against the Bruins was to have Pageau shadow Bergeron as much as humanly possible, and free up their top two lines centered by Turris and Brassard, along with Karlsson, to go against the Bruins other lines.
The Pageau line took a beating against the Bergeron line possession-wise, but Ottawa made up for it by their other lines carrying the play against Boston's other lines. Issue for Boston in the regular season was that the Bergeron line could not buy a goal at ES despite the massive territorial advantage. Bergeron still managed to net Boston's only goal against Ottawa at ES this season, but they still shot at just over 3% on the season.
I'm starting to think Marchand-Bergeron-Pastrnak may be a good idea for this one. Just try to overwhelm the Senators by so much in that 30% of the time they're on the ice that they have to adjust.