Well, the best way is to simply kill the Pursuit trapper once and for all. However, that is easier said than done, and just based on personal experience, the only thing you can do is a double switch. Send your Latias out and then immediately switch to something that threatens Tyranitar. Killing an enemy mon with something like that Mega Latias you mentioned will almost always result in Tyranitar coming in and killing you which makes the part you said of not using Latias until the trapper is dead a very viable option. Entry hazards + consistent double switches are one of the best methods for dealing with pursuit trapping as Tyranitar would get worn down fast. This should put Tyranitar in range of your Latias' Aura Sphere or Mega Alakazam's Focus Miss (Focus Blast).
In the end, this is just the same situation with having your Ferrothorn or Corviknight avoid that damned Magnezone. You can never be truly safe and you have to constantly predict and double switch otherwise it's gonna trap you. While Pursuit still doesn't technically trap, it's still a similar situation and until you get rid of the trapper, there is no true way.
