Walkthrough
At the very bottom of the Seafoam Islands, you will find the Legendary Pokemon known as Articuno. Articuno is one piece of the Kanto Legendary Bird Trio, being a dual Ice/Flying Type Pokemon. It will be found at Level 50, knowing the moves Mist, Agility, Mind Reader, and Ice Beam. Because Articuno is an Ice Type Pokemon and the only move it knows that actually causes physical damage to your Pokemon is also Ice Type in nature, Fire Type Pokemon do especially well against it. Since Articuno isn't the strongest Legendary Pokemon you will find in the Kanto Region, stick with using Ultra Balls to try to capture it.