Disable Sunset Invasion after starting a game

Such a vast empire is bound to get split at some point, particularly after succession. You can either force it by causing.. an unfortunate incident, as Kyle suggested, or sowing dissent with a powerful king-level or powerful duke vassal of HRE who dislike their current liege.

Of note is that you can beat larger armies than yours with some clever warfare tactics. You are a pagan, and if the enemy hasn't researched Military Organization at least two or three times (unlikely, if you are that early in the game), then they suffer severe attrition in your territories. Also important is that hauling such large armies can take time, and would give you a few precious months of consolidating your troops and destroying some of their armies while they call their Occitan legions. Hopefully your realm is much more compact than theirs, and have ample naval access for quick transport on sea rather than by land.

If you deliver significant damage to them, factions within their empire might grow powerful enough to cause a civil war. This can give you a significant advantage and might cause them to ask for white peace to deal with troublesome vassals. This will give you some ten years of breathing room for you to expand and grow until they are able to strike again.

As you always have a casus belli against them, yielding some lands to them now without a fight might be a good longer-term strategy to reconquer them later when they are embroiled in another conflict -- particularly if you don't hold any of the counties in the duchy.

You have two major options here, and one is much easier than the other:

1. Take a title, release him, let him rebel, imprison him, rinse, repeat. Eventually, you've taken so many titles that he isn't a Super-Duke anymore, and when you finally take his last title, he won't even be a noble anymore. This may take a very long time; probably so long that your Duke dies of old age and you start dealing with his heir instead.
2. Who said you have to release him? Just let him rot. He can't raise a rebellion if he's in your dungeons. If his problem with you is personal, not political, then stick him in the oubliette so he'll die sooner, and you can deal with his heir instead. (If his major penalties to opinion are things like "Ambitious" or "Greedy vs. Honest", then its' a personal problem and his heir will probably be different – though you should examine the heir to be certain. On the other hand, opinion penalties like "Too many duchies" and "Desires that shiny county you've got" will be inherited by his heir.)

As a side note, if you assign your Spymaster to seek out plots in an unhappy noble's capitol, you'll probably find a plot against you. Then you get the option to arrest that noble, which might or might not work depending on your relative Intrigue skill. If it doesn't work, he'll rebel immediately, and then you can crush him like the bug he is and look at those above options.