I'm a little conflicted on the whole situation - I mean, blowing up the death star is a really big deal in the Star Wars universe, it was tough, risky, and was hard to pull off. It makes sense that the same would apply in the pinball machine - - a mode that's tough, risky, and hard to pull off.
At the same time, it's basically the only toy in the game and it'd be nice if that major toy actually did something more often. The mechanism was probably a large part of the BOM, would have been nice if they provided more of an end user experience for the price they paid. Maybe it was just a bad design decision - - could've put the money somewhere else into a mechanism that would have made sense to activate more often and just had a simple Death Star with LEDs simulating the explosion. Besides, it's not like the actuation on the toy as is, really makes you feel like it's exploding anyway... A Slave 1 that moved some how every time you made a boba fett combo shot, or light sabers that lit up when you made a key shot, or R2D2 in the game where his head spins and lights up when a specific shot is made, etc. all might have been cool toys that could be implemented often in the game and made you think, yeah, that was worth the cost in the build of the game...
Anyway, as others have mentioned, it's easiest to blow up via the Endor route and be sure to light your add-a-ball. If you really are having a hard time, you might want to think about being more strategic in your game play and try to bring another multiball into it at the right time - planning ahead and starting Battle Above Endor with only a handful of Tie Fighters left for TF MB gives you the chance to bring in TF MB at the right time, adding a ball save, and then you can bring in your add-a-ball when down to the wire and really give you a good crack at it.