It took me a few attempts (4) against him as well; admittedly I'm Hume and not a Taru though. Might be worth focusing on a few HP+ items if you're dieing too quickly. It also depends on what strategy you use. If you ignore the book (which is what I did) you can defeat Gunther in 3 nukes (assuming no resists), and the fight is then not quite as dependent on the element of the Grimore. Use your Blink Band for a bit of breathing room, and just go all out as soon as you get into the battle; it stops Gunther having time to buff and you having to waste casting time with Dispel.
I posted my experience here
on FFXIclopedia; again I realise it's for a Hume, not a Taru, but with Taru's better base INT stats, pure nuking on Schultz may be the better option since you're less likely to get a resist.