Pokémon Gold and Silver/Return to Ecruteak City

'''This is a point in Crystal where you can take a little break from the main quest to go and get Suicune. Take your Clear Bell, your strongest Pokémon, and the best Poké Balls you can get your hands on, and Fly back to Ecruteak City, where Suicune has made a home in the Tin Tower.'''

Ring the Clear Bell
You couldn't get past the monk in the building next to the pond the last time you were in, but now you're packing the Clear Bell. He'll get out of your way when you give it a jingle, and you can then take the underground tunnel to a building north of the trees.

The Monk ambush
As is so often the case in these troubled times, you can't make it out of this dark tunnel without being ambushed by a pack of rogue monks. You'll need to beat all three before you can pass, but their level 32 -forms and s shouldn't give you too much trouble. If they do, you may not be ready for Suicune.

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The Battle with Suicune
All three Legendary Beasts are waiting in the Tin Tower, but only will attack you. This time it will fight to the finish instead of running away, so be careful not to K.O. it. If possible, use a Pokémon with False Swipe (like or ) to get it down to 1 HP before you break out the Ultra Balls.

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What about the rest of Tin Tower?
Even after catching Suicune, the monks won't let you proceed any further. To get their permission to pass, you'll need to prove yourself yet again by catching and  as well. And that means you must catch them in the wild—you can't fool the monks by trading for them. When you've caught all three, and defeated the Elite Four, they'll give you the Rainbow Wing that makes a level 60 appear on the top floor.