Your best bet on getting another one is to coordinate with a local group. The Silph Road subreddit has put together a map that will help you find the closest discord group to your area. These servers will typically have an EX Raid channel, as well as other channels, such as one for ditto spotting. I only learned about this map in the last two days, although I've been on two different discord groups in as many months (one in CA and one in PA).
However, we also know some of the rules by which Niantic uses to give out EX Raid passes, but there's still some guess work. Niantic themselves have said that battling and raiding at "sponsored gyms and gyms in parks" as well as having a high raid frequency and high gym badge ranking will increase your chances of getting an invite. How much this affects your odds, we still don't know.
What we do know:
- Head over to Pokemon Go Map dot info and click the settings in the top right, visual tab, and enable the level 12 S2 cell overlay (although one source I found said its level 13 cells, but all others agree that it's level 12).
- Only one gym inside each of these "super cells" can be EX-eligible in each "wave" of invites.
- This site has some information about waves. It does not appear to be regular, but it is roughly once a week with a rotating day-of-week.
- Gym must be located in a park (there's an overlay for this too, but moving and zooming the map causes a lot of requests and you can get timed out). The standard map view is usually accurate enough.
- You must participate in a raid at an EX-eligible gym
- That gym must have a certain number of raid participants.
- It still isn't clear how many or over what time period. The best guess is "about 20" but it doesn't need to be the same raid instance: multiple groups can raid separately and it still counts. One source lists as low as 14 unique players.
- The location of the EX Raid will be at the gym that made you eligible to receive the pass for and for a data and time 7 to 10 days in the future.
Even after all of this, not all trainers that participate in a raid at an EX-eligible gym will receive a EX Raid pass. The statistics work being done on the influence of gym badges and so forth doesn't yet have enough data, and that data collection may be flawed (as people who do not receive an invite are less likely to report).
Different strategies are around, but the one that makes sense to me is to just raid every gym within a given "super cell" at least once. However, coordinating with your local group will be important. They will likely already know which gyms can be EX eligible and have a strategy for raiding successfully.
One interesting side note: if you ever get a shiny raid boss, the catch rate is 100%. Toss a pinap at it and throw clean; easy 6 candies.
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