I definitely know that strategy.

I got some good practice going after the only Unova roamer in Tornadus (Thundurus in White). I knew I only had 5-10 minutes by the time I unlocked his reign of - uh - terrorific weather (it moves to Routes 12-15 at midnite), so I went just for weakening without thinking. Overnite, I realized Watchog still knew Mean Look - something I had somewhat planned on but not thought about until that moment. It also knew Hypnosis and Super Fang. A perfect combination.
It did get knocked out once (and predictably, after the knockout, the roamer resumed roaming). The second try turned out much better, lowering to the single HP and - because it was dark - snagging in a Dusk Ball. At least, I think I used that, I'd have to go back and check.