"They came up with a really good push, [Ullmark] allowed us to weather the storm and start going back at them," Bruins coach Jim Montgomery said. "He's been doing that for us all year. It's amazing how calm I am when I see scrambles in front of his net because he's calm."
DeBrusk made it 2-0 at 1:52 of the second period with another power-play goal, finishing a three-way passing play with Dmitry Orlov and Taylor Hall.
DeBrusk had left the game momentarily in the first period to get stitches above his left ear after being hit by a puck.
"Dusty, one of our trainers, kind of freaked out a little bit because there was a lot of blood," DeBrusk said. "Thought it was the ear but it was a little bit higher than that. But it kind of woke me up honestly."
Tkachuk cut it to 2-1 at 16:00. He got to a loose puck near the right post, put it between his legs, and scored into an open net.
Bertuzzi responded for Boston to make it 3-1 at 2:26 of the third period, tipping in Brandon Carlo's shot from the point.
Sam Bennett cut it again to 3-2 at 6:11 with a rebound on a power play. It was Florida's first goal in 10 power-play chances in the series.
"I loved our start," Panthers coach Paul Maurice said. "The game got away at the end by score, but not by scoring chances necessarily. We'd given up 11 even-strength shots through two periods, and the other guy made a couple of real good saves to kind of keep us from getting to be tied."
DeBrusk extended the lead to 4-2 at 8:05 when he poked in the rebound of Pavel Zacha's shot.