Oh dear this one can be problematic.
You need to win unifier victory which means that you must build a tower and ignite a flame on it (which counts as two separate buildings) but in order to do that you need to reach level 3 of race governace. Not only that but when you build your towers and start to ignite the flame (which can take a few turns at minimum) your neighbours WILL declare war on you if they are not allied and a top of that every two/three turns three stacks of enemies will spawn near that city to destroy the tower. So how to do it best? Here's my proposition:
1) When generating a map pick the bigest option and set the distance between your oponent as far as possible. Also make the AI dumb. And set the option to spawn lots of independent cities.
2) Check that you have vicotry by unity marked and set it to lowest possible. Why? Because you can build tower when you reach level 3 od race governace and you can only build 1 tower per race. So you need to reach level 3 for only two races.
3) Pick from the start two cities where you will build the towers and start to build garisons and fortyfy them. As I said previously there will be spawn of 3 stacks of enemies which will attack that city (and they all are about one turn to reach the city). And believe me they are no joke. They are the primary danger.
4) Construction of the towers is quite simple. Nobody will attack you when you are building the towers. Use that time to build your armies around them.
5) Now you have to ignite the flame. It will take some time. I recommend on igniting them at the same time because when the city have tower with already ignited flame the spawning of attacking armies won't stop.
6) When the flame is ignited on both towers you will win and achievement will be yours.