Need help snagging Entei. How do you do the secure ball trick? When do I press B?

Ok, I have snagged Entei in a regular Poke Ball many times. The only problem is that last time when it was successful, it turned out to be a Lax Natured Entei and had 154 Hit Points. I was mad because I wanted it to be Brave Natured. I weakened it and everything. I even had Typhlosion lower accuracy on it and Skiploom lower speed, use Sleep Powder and Mega Drain to further weaken it down and it still breaks out and then if I wait too long to snag it, it uses Shadow Rush on one of my Pokemon to faint itself. The problem is I may need another strategy, but how? Camerupt is always out when Marshtomp faints and the Pokemon that is using the Surf move is usually slow and is holding Quick Claw; her name is Quagsire. Surf hits both Pokemon after Jumpluff uses Mega Drain on Marshtomp a Water and Ground type like Quagsire to faint him and I try my best to get Entei to use moves other than Shadow Rush or Fire Blast. So far he has only used those 2 and Entei also has Bite which would happen if I sent out my Espeon and Stomp which he rarely uses on me. I tried a load of button combinations and they didn't work this time. Including pressing B right when the Snag Ball was about to open.