I think that just before battle you should get an armor (everytime I got for
Lulu) with magic absorb abilities so with her in party he can't do anything
to you

Try Nulfire, NulTide, NulIce, NulBolt.  I think those were useful in every
battle against Seymour.

If you mean the one on Gagazet, then there is no real strategy. Just use
all your summon's overdrives, and make sure that when you get zombied,
cure it (otherwise he will full-life you). I thought this one was
probably the hardest boss in the game. (maybe excluding the
second-after-next one (trying not to spoil things))

Hum no real tactic for me. Mostly ace attacks and summons and usually
when a character is knocked out I Phoenix Down them and
tag them out (because I'm too cheap to use MP or items to
refill their HP)

Does anyone else here just sit for an hour or four just fighting boring
random battles and levelling up? I do... kinda ruins the challenge of all
FF games abit...