One way is to beat them in transition. Forcing turnovers would help there too. It's how we won our lone regular season match up against them when Cleveland had their old roster (Wade, IT - injured, Rose, etc), and it applies here too.
Toronto actually did a good job against them in transition or when they went up-tempo trying to push the ball up the court. It's primarily how they stormed back in Game 3 that series before LBJ ripped their hearts out again. Indiana did a good job in transition as well.
Yes, the Cavaliers did get a bit younger with their flurry of trades (adding Hood, Clarkson, etc.) but this C's team IMO is faster and also younger. I'm not saying we keep running in transition and just jack up 3s, nor am I advocating that we erratically try to push the ball up the court prematurely (forcing it too much), but the Celtics have looked like a great team in transition all year and they were in the Sixers series as well.
I'm sure our defenders will hound them all series and force some TO's. Then, we got to run and attack.
Lebron won't tire, but we can certainly tire out much of their team if we attack more in transition and stay aggressive.