Through 20 minutes Montreal held a 16-6 advantage in shots.
Ottawa opened the second period with another early goal, scoring just 44 seconds into the frame to retake the lead. Austin Watson deflected Nick Holden's blue line shot to beat Carey Price glove side.
The Sens then scored three goals over a span of 67 seconds to jump out to a 5-1 lead before the game reached its halfway mark. Parker Kelly and Josh Norris scored at 6:43 and 7:15, respectively, before Kastelic added his second of the night at 7:50.
Cole Caufield got one back at 8:43 to reach the 20-goal plateau in his rookie season. Caufield took a pass from Mike Hoffman in the slot and beat Forsberg stick side to make it 5-2.