Medieval Dynasty

I finished the game at level 7 out of 10 so a nice exploit to knock it out would be to take a large stack of resources (I had 2200 logs, 11k coins, and 3/3 in barter) and sell it all to a trader, then buy it back, sell it again, buy it back. rinse and repeat until you get it. I reloaded a save afterwards as you lose a ton of money doing that with the price differences. It popped for me without even exiting the trade menu (with the xp set to +1000% of course). If you are not that far you could combine this with the achievement to turn in 1000 logs.

This can be made substantially easier by upping the EXP gained prior to beginning a new save game. Naturally putting it up to 1000x exp will make this achievement substantially quicker; Diplomacy is pretty slow at first so take that information as you will.

Diplomacy levels up by...
- Completing quests especially side quests
- Successfully flirting with partners
- At a Trader, each item you trade gives Diplomacy skill equal to 1% of the trade value.

The first perk in the Diplomacy skill tree will also add 5% extra Diplomacy Exp to all sources, up to a maximum of 15% (5% per Perk Point)

Naturally in the early game you won't be making a lot of money and quests will be slow due to not having the stamina/mount necessary to travel quickly. Later in the game when you have access to more resources you will likely find diplomacy becomes significantly easier to level.