A box end wrench with 12 points may round off the hex on the bleeder nipples. A tubing wrench with its thicker walls usually works better. A 6 point socket is a good choice to get it started. Soak the threads in penetrating oil first, and give it some time to soak in. I like Kroil or B'laster PB. Try really hard to avoid snapping off the bleed nipple. If you do, it's possible to drill & tap it out, or just get rebuilt calipers.