To get there, fly to Viridian City and take Route 28.;
, Make sure you have at least 10 Repels. Preferably, Max Repels, since it lasts for 250 steps.
 Make sure you save before battling it, otherwise, you'll have to defeat the Elite 4 again before you get another chance at catching it.

Go to Mount Silver.
Navigate into the cave.
Go into the cave, and you will find Moltres.
Prepare for a long battle!
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