First off, there is no way this Royal Guard is as powerful as you've said. If this is the case, then it's an item they put in the game so you could frickin' God-Mode. It has to have some limit or side-effect.

Secondly, Force Lightning is an attack that is blockable, but how would Dante block it? He's never seen an attack like this. It's kinda like the Sigma argument. Sigma hasn't got defense against Mana and Dante doesn't have defense against the Force because he's never fought an oppoenent before with the power.

Third, a lightsaber might not be able to cut through Dante's weapons, but could Dante's weapons destroy it? I don't think so. So, we're left with they can both defend against each other. Now, Palpatine has the advantage of being able to see the future. Does Dante have that skill? Now, I'm not saying that Palpatine is invincible in a duel, but it gives him the edge.

Finally, Palpatine can use items to block the bullets as well.

Of course, there's always the good old tossing of the lightsaber, with Force-guided accuracy. Then, he can pull it back.

I'm not sure about this one.