Well, first thing to do is to find out what units the superpower is using. To have a chance at beating them, you'll need to make units that specialize at defeating their units. For example, if they use lots of medium tanks, then you'll want to produce anti tanks, tank destroyers, and attack bombers. DON'T FORGET RESEARCH!!
But to mass-produce units, you need a good economy and lots of resources. Build industries and industrial complexes and level them up, and if there is a resource you are short on, use the stock market, or better yet, trading with your allies.
Next, you'll want to get a couple of other countries to help you with the attack. Since the superpower (probably) has a strong army, they would be able to easily defeat you alone. Getting 2 or 3 others to attack the superpower makes him have to fight across multiple fronts. Make sure you have share map with them, or at least right-of-way.
If you have money to spare, you might want to dispatch a few economic sabotage spies in his core cities or other cities of his that have high-level unit production buildings, further slowing his production.
When you start the attack, the first things you want to do are 1) take out his main (strong) units, and 2) capture his key producing cities. If you do so, it will halt his production of both units and resources. Air superiority is important, so make sure to be producing lots of interceptors and tactical/attack bombers, depending on what kind of units the enemy is using. If the enemy is using naval units, then you should produce units to counter their units (ex. if they have lots of battleships, make submarines & naval bombers)
Keep attacking the enemy's units and taking their provinces, using your normal attack strategies, and tailoring them to fit the current situation. Remember to communicate with your allies - you might want to make a coalition, or join theirs, if they have one. Once it seems that you have taken out most of their units, capture their remaining cities, and any weird provinces you didn't take. You might want to communicate with your allies and discuss who gets what territory, to make sure nobody isn't satisfied. If you have made a coalition, you might have enough VPs to win the round, if you don't, keep attacking other countries with your allies until you do.
That's pretty much it 
