Don't get discouraged, it takes time to get a unique kill. I had a similar grudge war going with uri...hunted him since L43 or so on my glavier, and didn't manage to kill him until... L59. I could finally outdamage the 6x only because I found him early and was able to get enough damage in to get the kill.
Worse thing with Isy is not really her, but her spawns. They pwn anything below 65. LoL, I've even went down a few times at 66 b/c of them. 3 words - elite white spiders. Ow.
You really don't stand a chance of getting Isy until you get 8th degree weapon, and have a party of guild/uni members to help you get kill! I've got her twice - L65 and L67. We had a union party (3 of us) that banged on her. The second was a non-party effort.
No way to deal a decent amount of damage without 100% ice resistance, those spiders are just too pesky. Make sure you have that flame shield buffed and your jewelry maxed for ice resistance.
It'll happen for you. If anything go through the motions and try to find her spawn points and the quickest way to search them. Remember places she is more likely to spawn and
play those spawn points first - unless you have a full 8 man xp share party you can't possibly cover all the bases by yourself.
Hunt in off-peak hrs. I got the 2nd Isy kill because no one else was looking for her.
If you catch her off in one of the non-ice spawn points, get her off of the road quick! Hit her enough to where she follows, and run as far off of the road as she is willing to follow. This 1) keeps others from easily finding her 2) buys you time for your party to arrive.
Good luck, and hope you get her and get nice drops too

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