I'm trying to get Illusen's staff, or at least a trophy, and I was wondering if there are any lists of items she asks for on the higher levels, or if any body has noticed her asking for one type of item more often than others. Is it just any r99 items or has it been narrowed down at all to a slightly smaller group? I'm also open to any and all advice for being more successful on the high levels because I've looked at a couple guides and still feel woefully unprepared.

The guides will help you to a point. If you want a trophy, currently you need a score of 15,273 just to get on the High Score Table. I don't know if there's a list of items she asks for (I'll let you know if I see one) but I do know that the higher you go, the more expensive the requests get. However, it's also a little random and she can ask for something really reasonable at a higher quest.  Many people do her quests up to Level 20 for the avatar then stop because it gets more expensive.  Good luck getting the trophy and have fun!