How to catch Miltank
Miltank is offered in different ways during the Johto Collection Challenge event. The Pokemon will appear in the wild more often, but the rate will still be lower than most other boosted characters and might escape those who don't explore the area too much. Instead of just finding one, there are two more ways to try and catch Miltank before the collection challenge ends.
Miltank will also be featured in 3-Star Raids at Random Gyms throughout the event, which runs January 26-31 at 20 p.m. ET. Be sure to check your local gyms to see if there are any. If so, take a few friends and eliminate them. As a Normal-type Pokémon, Miltank is vulnerable to Fighting-type attackers, so use your most powerful and you shouldn't have much of a problem defeating it.
Otherwise, you're stuck with field research tasks, which are even more random than the methods listed above. The one you're looking for is Using 7 Berries to Help Catch Pokémon, which will always reward a Miltank. Now that you know how to catch one, you might be wondering how to get a Shiny Miltank.
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The Brilliant Chances for Miltank are increased throughout the Johto Celebration event, so the more you encounter, the better your chances. While you only need them for the Johto Collection Challenge, you'll want to meet them all you can if you want the newly released Shiny Miltank.
All of the above methods have a solid chance of being brilliant, so focus on the ones that are available to you. Raiding and research can sometimes have a better chance, although we do not have confirmation yet. So the key is to just meet as many as possible while the event is live and the shining odds are at their peak.
And here's how to catch Miltank for the Johto Collection Challenge in Pokémon GO, and some tips for getting Shiny Miltank.