How to Win the Korean War without paying 30 Tribute

The key to this mission is your Navy and economy.
First pause the game, and work to boost your economy as best as you can. Oil and Wealth will prove extremely useful late game, so its helpful if you send some citizens to build oil wells.
Researching Commerce and Military early on will greatly benefit you, allowing you to boost your population and resource income. Civic research can take a backseat, and is not a priority. Scientific research will play a significant role in building missiles.
Regarding defense once the match starts, send the air carriers to boost the South Koreans in their defense. Air superiority will weaken the invading forces. Do not be too worried about the enemy seizing Seoul, you can always take it back. In regards to your infantry (Both your army on the mainland and your stronghold), sending them to face the enemy army is ill-advised, since they will be slaughtered. Hold your troops back, or send them to garrison some of your allies buildings to assist in the defense.
IMPORTANT: With defense settled, this is where things get crucial. At the start of the game, send your battleships and produce more to attack the enemy's docks before they can build ships. At the same time, send a citizen to build a dock in Daejon, and produce ships to take out the enemy docks on the left. Naval superiority is key to winning the game. Your battleships can wreak havoc on the enemy cities, allowing your troops to quickly capture late game. Have your ships bombard the enemy cities, while ensuring that they don't build new docks.
When possible, send some workers to build a new base camp, and airport on the other side of the coast.
Spend the rest of the game boosting your economy, and building up your forces. Infantry is key to this mission, I would not be so focused on building tanks.
Now, here is where science comes into play. While your Navy can handle most of the outer cities, there are a few mainland cities, including Pyongyang.
The moment you seize these cities, the Chinese will send their armies complicating your war efforts.
STRATEGY: The Main goal is to use the last few minutes of the game to quickly rush and seize all the cities simultaneously before the Chinese can react.
Build up a set of missile silos and aircrafts, and bomb the enemy cities, weakening them. Your infantry (and armor if you have any) can meanwhile focus on retaking Seoul (if captured), and eliminate the outer air defenses.
Assuming your battleships on both flanks continue bombing the enemy cities, send a few infantry groups to the sea, holding position just nearby the enemy cities, but don't let them land. One infantry group on the left for Chongju, and two-three for Wonsan, Iwon & Najiin.
During the last 5 minutes before the timer ends, you should have built a strong army, and most of your enemy defenses are in ruins thanks to massive air support and missiles.
OPTIONAL: If you have the resources, you can send some helicopters to secure the inner cities and deal with any infantry groups for your final push.
Make sure that the enemy cities health are reduced to zero and are ready for capture before you attack.
Send your main infantry push to capture Pyongyang, while at the same time sending your single infantry groups in the sea to seize the outer cities. One group for each coastal city is enough.
China will then send their enemy forces, so rush your army to take Yangdok before their armies can respond, the deepest enemy city in the mainland, and win the game. Timing is crucial, for the enemy reinforcements will eliminate your weaker infantry groups.
CONCLUSION
And with that, you have successfully won the mission, and gained both Koreas as client states without paying the extra 30 tribute. I hope this guide has been helpful, and will contribute to your campaign.