Step 1. be a small faction like the franks
Step 2. burn your homeland and invade the british isles
Step 3. peacefully sit on the british Isles for around 70 turns and build a super economy
Step 4. by about 420, everyone except you and a few stragglers will be blazed by attila or they will have moved far south
Step 5. use all the cash you stocked up to colonize the ever loving ♥♥♥♥ out of the ruins of northern europe
Step 6. build a lot of goat pens
Step 7. you are ideally at this time making about 30000 a turn and you havent been at war with anyone for like a hundred years
Step 8. build a massive ♥♥♥♥♥♥♥♥ army with your huge bank (6+ fullstacks)
Step 9. move your super horde south and arrange your stacked armies at the capitals of all the surviving nations (who are all ideally still at peace with you at this time)
Step 10. kill all of them
Step 11. find attila and push his sh.it in with a army made up of artillery, heavy spears, and a big ol pile of cav.
Step 12. win the game


notes:
attila often wont attack you if you dont attack him
ignore his enchanting raiding dance and stay out of his attack path for as long as possible (southern france, italy, and south east germany) until you find attila himself

winter is your ally
if you find if problematic due to attrition and foodshortages
just give your armies the resistance to cold tradition
and just ♥♥♥♥♥♥♥ learn to deal with the lack of food surplus
you get more than enough if you manage your buildings properly