If you don't have a way of keeping it in the fight then use a weak attack that always goes first like quick attack, then when Mesprit's health is in the orange/red just use an ultra ball, quick ball, or a dusk ball. I find that dusk balls are far more effective, i'm saving my master ball for when ever i get a chance to get the azure flute event (I doubt another chance will appear, oh well).
I love how the internet is its own fantasy adventure novel. Person A seeks guidance and the Trolls devour him.
i try not to use a masterball but ive got my way to catch it but involves having to track it down over and over again.
when you first see it paralyze it
then try to get its HP as low as possible (using false swipe is a good idea)
then get a pokemon with the ability intinmidate (lowers attack giving you better chance of catch) and keep using the quick/fast ball (whichever one works best when first used) repeate this step if catch fails
I am a pacifist which means i will PASS A FIST through your face
I caught Thunderous from pure skill, but he's easy, since he stays in 1/3 routes. But he runs every turn too, so I chased him! Tips? Don't use a master ball, especially if you want to brag about the capture. Capturing 'runners' takes time, so keep tracking the pokemon as well. Good idea to keep a sleep inducing pokemon in your team while intent on catching the runner, false swipe is nice too. Use the sleeping move, then repeat using false swipe until 1hp. Then keep throwing balls + using the sleep move, no matter how many times it flees! BOOM.