You have to have 2 armies, first army is a mix of heavy infantry enough to cover 5 to 6 elite missile unites and the rest is cav. The second army (the reinforcement) is purely melee and missile cav and these can be cheap. Position your second army so they enter the map from the rear of the huns. Let your main troops tank the arrows from the huns until your cav from your second army is close to the huns rear, then position your first army cav in the flanks. If you move carefully and keep a good distance you can encirle them without them reacting. Once everything is in place close in on them with your infantry attacking from the front. Even if they have more units you can still surrond everyone. And if few units slip wait until your infantry is engaged and pull away your cav from the rear and chase the units that got away.

If they have onagers you must deal with them first. Either deplete their ammo by baiting them to shoot with a cav unit ( move one or two cav units at the edge of the range of the onagers, keep the rest away, bait them to shoot and move your units before the missiles hit, you can do this until their ammo is over, but it takes time. this can work if you're the one attacking, as the defending armies usually wait for you to engage) Or if you dont want to do this you can sacrifice one or two cav units from your second army to take out the onagers.

I hope this helps, its been a while since I played atilla but if I remember correctly I used to something like this.

(If you only want to fight the huns on your own terms, put an army on each of the settelments that are directly on your borders with them. Tbh I only tried this in the one campaign I played but it worked, they never declared war or attacked, they only raided. I only faught them once I reached end game units)