While it's true that you can catch most Pokémon with an Ultra Ball, the chances of successfully capturing a Pokémon depend on several factors, including:
* The Pokémon's level: Higher level Pokémon are harder to catch.
* The Pokémon's health: The lower the Pokémon's health, the easier it is to catch.
* The type of Poké Ball: Ultra Balls have a higher catch rate than regular Poké Balls.
* Status conditions: Certain status conditions like "Sleep" or "Freeze" make a Pokémon easier to catch.
So, while it's possible to catch Suicune with an Ultra Ball, it's not guaranteed. You'll need to use strategy and some luck to make it happen.
Are you playing a particular Pokémon game? Knowing the game and its mechanics could help me give you more specific advice.