This depends on a ton of factors including level your playing at but it looks like:I've got a pretty specific question and I'm not sure where to ask but:
What is the minimal Attack EV investment an Impish Tyranitar needs in order to OHKO Latias/Latios with Pursuit on a switch?
I can breed you the Scraggy you need, PM me :)
140 Atk Tyranitar Pursuit vs. 0 HP / 0 Def Latios: 300-354 (100.3 - 118.3%) -- guaranteed OHKO
Though I simply doubled the attack power so it may be incorrect