No, Scary Face is not a trapping move - it only decreases a Pokemon's speed, so it won't prevent Suicune from fleeing.
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You can use a Move like Scary Face...
If its a Pokemon that runs away like entei, raikou and suicune weaken it when you find it when it shows up again it will still be weak now throw a ultra ball if you fail keep trying!
scary face golbat and zubat can know it hah trying to keep latios or latias from running away
It will keep fleeing unless you have a Pokemon with Mean Look or something like that so it won't escape. Try catching a wild Golbat. The same is for Mespirit.
If I want to get a shiny Pokemon then I use the Action Replay/Game Shark and Cheats but if you can't get it or something then when you are about to battle Suicune then save before you do and start battling him then if he is not a shiny Pokemon then just keep on trying.