If you want a diplomatic victory disable espionage because coups are currently broken..
And if you want to have good friends avoid a lot of dows make sure you dow the right person
I would say maximum going to war twice...
Pick the persons who are hated by everyone... Proparly the warmongers
A good startegy is to settle near you're neighbour and turtle until the renaissance or industrial at this time. he will have dowed you constantly and proparly other civs will denounced him for backstabing you constantly. Make sure you don't have a strong military but a decent one to fend of AI's swarms.
After that pick a other neighbour who is hated or stay low depending on the situation..
Keys here is only pick targets who are hated
As great persons go try to manage them and get key wonders with great engineers. After that just spam great artists and great merchants
If you want to make you're game easier put 2 warmongers in the game and the rest all peacemongers and 1 average guy. so for example:
alexander , oda , ghandi , england , Siam , washington , persia( is fairly friendly and doesn't hate warmogner's that much except if you become reallly agressive).
All civs have a high warmogner rate except the warmongers of course so easy to pick a target
As a side note pick city states nobody likes first so you don't get we are competing for the same city state get the one's they like near the end game;