First let me me list some points:
1. Due to the cap of 50, sleepgaII will be unavailable to cast. This puts you at a bit of a disadvantage as sleep doesnt stick for very long. However, with three black mages that may not be much trouble.
2. Bring re-raise items. This is not so that you can get back up mid-battle. This is for if you wipe. If your situation looks grim, run far away from where the mithras spawn, and once they kill you (and everyone else) RR if they're far away enough. They don't have auto-regen so your only problem will be time.
3. With three black mages I'm not sure that using -ga magic is such a great idea. Unless they're galkas i guess. I've seen like two or three galka blms in all my time here though.
If everyone stays calm and does their job right, this should be pretty easy for you. SMN is not required, and my party did it without one, we just used the method listed in point #2. As the guy before me mentioned, if you have a PLD don't have him hesitate to use invincible. As for your black mages, freeze would work to quickly take out one of the mithras or pets, but they'll need to have either some really good meds or use their 2hr after that. Good luck to you.