I used an Ariados, Smeargle, and Parasect for Generations with Roaming Legendary Pokemon.

Ariados would use Spider Web, Agility, and Baton Pass.

Smeargle would have Defense related Stat Boosts and Baton Pass and Ingrain in case they had Roar.

Parasect would have Spore, Swords Dance, and False Swipe.

So you would prevent the roaming legendary Pokemon from leaving with Spider Web, boost up and pass off. If they had the ability to Roar, I would pass to Smeargle and Ingrain so I couldn't be forced to run then keep abusing Baton Pass to beef up Parasect. Gen2 I just used Fast Balls. Gen3 and on, I had my fleet.