One way to build suspense before a climactic battle is to give the PCs an inkling of just how badass and well-prepared the enemy is. You can leak this information to them in a variety of ways. They can steal plans detailing the enemies forces. They can scout the enemy base. Or, perhaps best of all, the enemy can simply phone them up and taunt them.

Transcript

Darth Vader: You two, follow me. Their old X-wing style fighters won't stand a chance.
Darth Vader: We'll be flying my personal enhanced fighters. Stealth technology. Advanced targeting systems. And, most importantly, the most competent pilots in the Empire.
Clone Pilots: Thank you, sir. {yellow speech bubble}
Darth Vader: Because I will now be Force controlling your every action.
Clone Pilot 1: If you want something done right, do it yourself. {white speech bubbles}
Clone Pilot 2: That's what I always say.
Clone Pilot 1: They shall fall before us like stalks of wheat.
Clone Pilot 2: They shall be ground in the mill of our might.
{beat}
GM: So... it's not monologuing when three people are doing it?