Let's look at possible matchups and how we have fared so far
Tampa Bay 3-1
Toronto 0-2 (one game left)
Carolina 0-3
NY Rangers 0-2 (one game left)
To me, the Bruins' biggest weakness this season is against teams with genuine speed and skill - the Leafs, Canes, Rangers and Panthers all possess this in spades and the Bs generally cannot handle it, as demonstrated by their negative records against all these teams except for Florida. I believe one of the main reasons for this is Boston are a team that likes to handle the puck and dominate possession, and they are not a slow team themselves. That's their style and their defense is setup accordingly, seeking to push the play and backing themselves to be able to scramble effectively to shutdown turnovers and transitions the other way.
The problem with that is when a team comes along that's even faster and more able to move the puck with agility and precision than them, they struggle to counter it and switch to a more shutdown method that effectively polices neutral ice and entries into their zone, and the goals against can pile up in a hurry. It's for that reason that I'm currently of the opinion that Tampa would be our best matchup, regardless of them being the defending champs, because they lack the speed and quick puck movement of the others. But the odds of that happening appear very slim, as is that of facing the second-best option - the Panthers. Florida have as good an offense as any but their goaltending is a massive question mark and the uncertainty over when Ekblad will return may also significantly weaken their D, so to me they're vulnerable. But again, Boston seems very unlikely to face them.
Which leaves Carolina, Toronto and NY as the most probably options, none of which I like one bit. All are beatable, especially the latter two, but each of them will be a very tough challenge for the Bruins.