Age of Wonders: Planetfall

In order to get this, you'll need a planet with some inherently high XP modifiers. However, you can raise the modifier further yourself by increasing your opponents level, and by changing the game intensity to a higher mark.

The modifier I would look for is "ELOP Alliance" on a 240%+ world. The ELOP modifier adds an alliance of 4 commanders to the planet. However, unlike "Marked for Reassembly" or similar alliances, they are not hostile by default. Once you are on the planet, explore around. Use influence to greet and compliment everyone you meet. Once you determine two alliance members, start continuously complimenting and gifting the member most amenable to you. After a while, you can raise your alliance level with them, and then do the same with each member as you meet them. Once you're in with them, just take out the remaining commanders as you are called into wars.This way, you basically make the modifier work for you, which is helpful if you're like me and can't take on the highest AI on your own.