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Just an FYI, you can catch a level 33 Gastrodon out of battle as soon as the game lets you roam freely, which also has the side effect of immediately leveling your team up into the 20's after you catch 2-3 of them. Once you gain your first single star rank (which is realistically when most people will actually start free roaming) you can immediately catch the lvl 33 Luxrays out of battle as well.
And given that the game has an ambush mechanic, it's surprisingly easy to get the drop on alphas, put a status effect on them, and throw a few balls, running, and rinse and repeating even if you need to be a higher rank to actually catch them out of battle like OP implies.
