David Pastrnak had two assists, and Jeremy Swayman made 34 saves for the Bruins, who are the first wild card from the Eastern Conference.
Bowen Byram and Peyton Krebs scored for the Sabres, who are the No. 1 seed from the Atlantic Division and rallied for a 4-3 win in Game 1 on Sunday. Ukko-Pekka Luukkonen allowed four goals on 19 shots before he was replaced by Alex Lyon 16 seconds into the third period. Lyon saved all seven shots he faced in relief.
The best-of-7 series is tied 1-1. Game 3 will be at TD Garden in Boston on Thursday (7 p.m. ET; HBO MAX, truTV, TNT, NESN, MSG-B, TVAS2, SN360).
After a scoreless first period, Arvidsson gave Boston a 1-0 lead at 4:54 of the second. Jonathan Aspirot lobbed the puck out of the Bruins' zone, and Arvidsson, out of the box after serving an extra penalty for Nikita Zadorov, beat Buffalo defenseman Mattias Samuelsson to the puck and slid a backhand five-hole on Luukkonen.
Geekie made it 2-0 when he flipped the puck on his backhand from center ice and it bounced over Luukkonen’s glove and into the net at 16:29.
Zacha redirected a feed from Pastrnak in the slot past Luukkonen stick side to push the Bruins' lead to 3-0 at 18:10 on the power play.
Arvidsson scored 16 seconds into the third period to make it 4-0, beating Luukkonen high glove side from the left circle on a 2-on-1 with Zacha.
Byram cut it to 4-1 at 13:54, taking a pass from Beck Malenstyn above the right circle and beating Swayman with a wrist shot over the glove.
Krebs then narrowed it to 4-2 when he backhanded the rebound of Rasmus Dahlin's shot from the point through Swayman’s pads at 15:08.
