This is going to be one of the more frustrating achievements to get in FFX. I had several attempts of failures between 180-199 before I finally got it. It really just requires patience and calm focus to be able to do.
This achievement has to be done in the Thunder Plains, and it is necessary to dodge 200 lightning strikes consecutively, without ever leaving the zone, saving/loading etc. Keep in mind that the achievement will not pop until you've claimed your reward, which includes swapping to Rin's agency in another zone. This means you will have to keep count yourself of how many you've dodged consecutively before going for the reward.
I recommend going to a spot in the southern part of the plains, where you can have complete control over when lightning will hit to make it more predictable. Once you hit your zen mode and keep at it, it will come eventually.
As you approach the crater, lightning will with 100% certainty come, if you've been far enough away. Dodge, go behind craters, dodge, repeat 200 times. I dodged around 500 consecutively in 35 minutes. If you set a timer for 25~30 and manage to not get hit you should be well above 200.