Recommend you have all Elixirs in your items. Try this combination for magic, Curaga, Aeroga, Thundaga (the third one doesn't matter, since he can't be hurt with magic much, but it's handy since it never misses). Never go to attack him without Aeroga active, and keep your Curaga shortcut handy... when he goes for Sin Harvest, open your item slot, but if you're close enough to hit, take a risk, and you can stop his attack... if not, back off, wait for the attack to hit, then use the item.
When he gets down to half health, that's when the pace starts to pick up. He goes faster than ever before, and catching him with more than one attack is a challenge.. still, he rarely uses Sin Harvest anymore, so that's always a plus.. unfortunately, his attacks get more speedier... just keep Aeroga up and ready that Curaga all the time, cos when he hits - he hurts! Oh, and he likes to port behind you when he attacks, so be prepared...
I have beat him - L83 on normal, L86 on hard... he's tough, but not too bad
Hope that all helps...
