Winter is coming! Drastic climate changes has left the world colder, with a devastating effect on once fertile grounds, forcing their inhabitants to leave their ancestral lands in search for new, prosperous lands. And there you are, the (Western) Roman Empire, still standing strong (maybe with a tad of help from my WRE early game guide), what kind of refugee could resist the safety of settling in a Roman province. What marauding army could resist the riches from sacking the greatest empire the world ever saw.
As you by now have survived the perils and pitfalls of the early game, kudos to you, that is no small accomplishment, the time has come to face one of history's most renowned leaders. Attila the Hun.
So far you have likely had very little contact with the Huns, as they generally spend the early game pillaging in the east and just generally ignore Roman territories, but that is about to change.
The Huns have amassed quite a few riches while devastating the eastern lands and fields strong armies with top notch troops. You might have gotten used a Roman advantage when it comes to soldiery, generally having far superior troops than your neighbours during the countless border skirmishes of early and middle game, well those good times are over.
The Huns are stronger than you in almost every aspect and are likely to come out at top in any fair fight, with stronger infantry, cavalry, horse archers and gigantic onagers capable of inflicting heavy losses before the real battle has even begun.
To add to your miseries you might have to deal with the Sassanid empire and their myriad of client states in the middle east who, by now, likely has overrun what remained of the Eastern Empire around the mediterranean, at least if you are like me and let the Eastern Empire handle themselves while you focused on your own internal challenges.
Of the two late game main enemies, the Huns and Sassanids, the Huns are by far the greatest threat and you shouldn't have any trouble holding the line against the Sassanids with more than two legions and a single fleet in the mediterranean.
Thus, if you haven't done so already, subdue any remaining minor nations in the Sahara desert and prepare to repel multiple stacks of invading arabs/persians. Your main concern is dealing with the Huns, establishing Roman rule over the middle east can wait.
So, about the Huns. As I said they have the advantage in any fair fight, which means that you want to attack with far superior numbers. To achieve this you will need to be able to field quite a few legions, duh. Level up our generals cunning to level 6, which allows for night attacks, and send a network of spies to eastern europe.
Reason for this is that the huns like to raid with multiple full stacks travelling together, everyone needs a friend, which makes them hard to deal with in open battle. But by engaging in night battles you can isolate each stack and overpower them one by one, rather than fighting everyone at once. This will lead to far fewer losses among your ranks and allow you efficiently deal with monstrous armies.
The spies will allow you to locate hordes before they reach your land and you can use their hinder army ability to slow down individual stacks, thus effectively splitting up the horde so you can pick them of one stack at a time. This also means that once Attila comes to age, you should be able to assassinate him before he even gets close to your territories, saving you a lot of trouble.
Once the Huns are dealt with you can send half of your armies to the middle east and steamroll the Sassanids, your infantry is stronger than theirs and you shouldn't have any problems subjecting the region.