I'm actually really liking the dragon tail set - Nothing on the special-spectrum is going to hurt it with assault vest, and with mixed attacks you can cover whatever your team needs to cover. There's fun prediction if you couple it with infestation, too! If you think they're too brainless to switch out their special attacker you can use infestation and then kill them at your leisure while they struggle to damage you. If you figure they'll switch, and you use dragon tail, not only do you immediately phaze whatever they send in (Unless it's fairy), but they'll know you have a phazing move. Nobody really wants to SWITCH if someone's constantly phazing, because they'll just lose their turn and probably rack up damage - if you phaze in something with special attacks, especially if it's something frail, that might be a good time to trap it if they think they want to trade an ice beam or a shadow ball for the dragon tail in hopes you'll phaze in something that can pound it.
Edit:
True, but a lot of other big physical attackers will do a -lot- of damage. Terrakion and non-mega Lucario's close combats will do over half damage, and you can't OHKO back. Sure, you can OHKO Gliscor or something, or put a dent in something else, but it's not worth the loss of longevity you get from taking out over half your health. If Goodra could get recover, I'd be a bit more open to a mixed set, losing the extra bulk of assault vest or something for the ability to heal off accrued damage.