With the WRE I usually destroy barbarian or nomad factions which can create a horde by leaving them with one single settlement and, after that, I assasinate or kill their remaining family members. If that's not enough, I assault their final city, kill their family members in battle and then withdraw. The city will turn to rebel, but instead of having a horde on my head, I have a strong faction destroyed.
Now, let me tell you which are the best tactics with the WRE.
First I leave all the cities which are useless (and the ones to which I can do nothing to stop their revolt) in the hands of the WRE rebels. Note: never leave troops in a city that you know it will revolt! They might betray!
Then I defeat the Allemanni, the easiest faction at my borders (it doesn't turn into horde).
Then I retake the rebel cities I once had (the ones possible).
Usually, after that, the Berbers attack me at Carthage with a huge army. As Carthage never revolts from the second turn, I succed to sally forth against them...
Note: Each time a settlement is gonna revolt, abandon it, wait for the rebels to get in, and then exermine it. (if you don't do that, your budget will be destroyed most times)
After this, you'll esily destroy the Celts, as they are a weak people (in BI, cause i don't say anything about their followers today...).
Now's the hard part: you've gotta destroy the Fanks. Send an assasin and do everything you can to kill most of their family members. Usually, now the Saxons will also send an offensive in Belgica. You'd better get some forces here, rather than sending them in Tribus Franki.In Franki send the Lily-Livered with some other troops and members to extermine the members left. Do not capture the city!! After you kill the members, lose the battle!
Next, some peaceful times will follow... Just before destroying the Saxons and regaining your power in the North.
It's time to retake Noricum. Much better if you haven't lost it till now, cause you'll need it against the Sarmatians, Lombardi and Vandals. Most usual, these 3 hordes attack you simultanously. If you have an army, victory is possible, because, after, besieging you a few times, the Sarmatians will go North in the rebelious Tribus Lombardi and settle there(most usual, but with few exceptions) and the Lombardi will go South, over Moesia and Illyricum. When they reach Italy, they'll be doomed. The Vandals will only have 1 or 2 horde armies and they'll be the most agressive. Their capital is usually Salona and after fighting you, they'll be destroyed by the Roxolani.
Around 395, the Huns will attack Noricum (you should better keep it because it's an important economic city). You should face them because they are weakened by the ERE and Goths, now settled in Iazyges.
Perhaps the biggest threat in Noricum are Slavs, which aren't to face. Usually they turn the Burgundii into a dangerous horde. However, by this time, you should have already dfeated the Berbers and the WRE rebels, such as launched an assault on the ERE in Cyrennaica and Egypt. Resisting a little in front of the Slavs to capture a few provinces in the East should ensure your victory...