For roaming legendaries that flee, I first Paralyse them with a fast electric type with Thunder Wave. Something like Manectric or Raichu works well. Then on the following encounters I use a powerful attacker with False Swipe. CB Scyther works, CB Scizor works, Thick Club Marowak also works, preferably high level. Once it's down to 1 HP I look for them again with Wobbuffet in the lead. Then once I see it I just start spamming balls. Arguably sleep is better than Paralysis as it increases the catch rate more, but the legendaries wake up from it again. Against the Lati twins sleep works best though, as they cure their paralysis with Refresh otherwise.
Master balls are the cheapest thing ever invented. It's more fun catching everything with balls that can fail. 'caught Latios in the wild without master ball ftw'