Farm PT Seal R.I.P.

[Mirage]

blu/nin + blm/brd covers most bases. Add a whm/whatever if you really want.

Personally, I didn't care about covering every single spell, and just went with somethingevasive/nin and blm/somethingwithcure. In the rare cases that we bothered waiting for a third, we'd get a blu or whm.

-edit-
forgot to mention that dnc/nin is good, because then the blm can sub brd instead of something with healing spells. That leaves out only blu and whm spells. Farming seals on days where you can't get light elemental weaknesses eliminates the possibility of the weakness being a whm spell.

This was relatively easy for me when I was at level 90-95 (haven't played since that was the cap), with pretty average gear. Now at 99, it should be practically impossible to die to most seal NMs if you know more than half of your job's capabilities.