I caught them all last night. I think the highest initial capture rate with a legendary sphere I saw was ~15%, with them down to super low health and getting stun batoned. The % it shows is bullshit, but I think in all the cases it took about 10 spheres so it's not far off. I definitely don't recommend trying one without 20 legendary spheres. You'll probably catch it in 10, but it'd suck to go home empty handed after ~10 minutes of fighting.The legendaries must be a giant ass pain. I went to try and do Blazamut with my best water pal being a 39 Jormund, and it took like 15 minutes. Had to use 3 legendary spheres, too.
Also bring poison and fire crossbows, poison/fire damage is % based and ignores defense, so it's very strong against legendaries.