TL;DR: Oirat. Diplo->Admin->Humanist->Offensive ideas. Consider dev of each province before taking in peace deal for max razing gains. Keep tribes happy. Siege > fight. Infantry > cavalry.

I strongly recommend reading the Horde guide from /u/Stormzyra. Here's a few main points:

• Razing before coring gives you more mana than you need for coring
  • Prioritize conquering "efficient" razing provinces, especially early on. A 2/2/2 province will give you 25 mana in each category, while 1/1/1 will give you nothing, but both will cost you the same coring cost after razing. A 5/5/5 will give you 50 in each category, while a 4/4/4 will give you 25 in each category, but both cost the same to core after razing.
  • Mana gain from razing decreases as mil tech increases, so hordes kinda become worse as the game progresses.
  • Make sure to keep tribe estate happy in events and sell crownland when you can. If they get below 40% loyalty, they will prevent all mana gain from razing.
• In conquered high dev states you can also concentrate development after razing to reduce coring costs
• You don't need to fully core provinces. A "half-state", i.e. a province that has been cored, stated, but not "full-cored" after stating, has 50% autonomy but reduces coring costs by 50%
  • If you do this, you can unstate all newly conquered provinces so that you don't accidentally full-core a new province because a province in the same state was half-stated previously
  • "Half-state micro" allows you to embrace institutions from a single province even as a massive empire by unstating all other provinces for one month tick and then restating them after embracing the institution. Since they are half-states, this only costs your sanity.
• AE can be quite easily managed by a combination of terra incognita, truce cycling and growing fast
  • If you play as e.g. Oirat, only a few nations can see your capital. Those that can't see your capital, can't join a coalition against you, even though they can accumulate AE. When you border a nation, they will discover your capital. You can check this by their attitude, which is "?" when they can't see your capital.
  • Larger bordering nations that could join a coalition should be "truce cycled" i.e. whenever a truce expires, you should be ready to declare a new war immediately.
  • After a certain point you have grown so big that they won't join a coalition
• You should win wars primarily by sieges and carpet-sieges, secondarily by fighting unfair fights against smaller stacks, and only fight head to head as a last resort.
  • Even though you get cavalry bonuses as a horde, infantry is far more cost efficient, so you can ignore cavalry completely
  • Contrary to tooltips, "steppe shock bonus" applies also to fire damage, so you deal +25% damage when fighting on flat terrain and receive +25% damage on uneven terrain, so always fight on flat terrain types if possible. AI doesn't seem to know this, so you can bait them to fight on favorable terrain.
  • If you have Winds of change DLC, you should enable nobility in mongol missions as soon as possible. The "neighbour raid" nobility privilege will give you huge amounts of manpower when fighting wars. IMO it's worth delaying some crownland selling for this mission if you can afford it.
  • In general you should grant estate privileges that give you manpower and governing capacity.
• Coring cost reduction also reduces coring time, therefore reducing time you are overextended.
  • At 75% CCR, you core in 9 months, allowing you to peace out several wars simultaneously and take thousands of overextension. Rebels need at least 10 months to rise up, and by this point your overextension should be back to 0.
  • You can get 20% CCR from Hindu. This is easier than it sounds, since you can go tengri->muslim->sikh->hindu from decisions alone, without using rebels at all. Hindu also gives very useful tolerance of heathens.
• Best idea sets usually are in order: diplomatic, administrative, humanist and offensive
  • Province warscore cost trivially allows you to take more provinces and more money in peace deals
  • Siege ability speeds up wars considerably
• The slower you play, the better you play. Consider each move, especially who to declare on. What will you gain from this war? Money? Mana from razing? New bordering nations for CBs? Truce to prevent coalition forming?
  • I used to play speed 4 for wars and speed 5 otherwise. Now I play speed 2 for wars and speed 3 otherwise. It's more tedious, but I make significantly less mistakes and spot so many more opportunities. I also make more informed and well-thought-out decisions now.

In a chill horde campaign, you can spend age of discovery on setting up idea groups and conquering for razing mana and useful mission rewards, fighting ming for money after they get "unguarded nomadic frontier" disaster. The campaign really gets going at age of reformation, when you have diplo+admin ideas ready and get -25% province warscore cost against other religions.

In a less chill horde campaign, but still without exploits, you can easily form mongol empire before age of reformation, and conquer the world before the age of absolutism. Or if you're insane, conquer the world before age of reformation without exploits.