It might be possible in something that can wield a disruptor/pulse turret though such as a bird of prey. But it would take a while.
Basically, fly around in circles outside of phaser range of the big ship.
If you have 2 disruptor turrets and a cloak equipped this is better because you'll do increased damage, and if your health gets low you can cloak, head to the planet and repair - planetary access might help here as well.
But its not actually invincible.
I suppose another way to do it would be to make a plug-in and give the sabre a million shields. But it would still take a while to cut through the ship unless you upgraded the weapons as well. But then it wouldn't really be a sabre would it?