I expect this to be harder than Raja, and quite possibly the hardest solo overall (though Seth and Gryz would also be quite difficult, Seth because of his lack of techniques, a hard limit to skill charges, and his inability to wear heavy armor or shields, and Gryz because of his poor magic defense, factor 5 weakness, and lack of any non physical skills or techniques besides Brose).
I'm at level 22 right now, grinding in the Ladea Tower to get ready for Gy-Laugiah (who won't be nearly as bad for Rune as Raja). Here's an analogy for Rune vs Raja: Raja is basically the Floyd Mayweather of Phantasy Star IV. His battles are a cure for insomnia, he doesn't go on offense much, and it's really, really tough to knock him out if you can't hit him with a haymaker. Rune, on the other hand, is more like...I don't know, maybe Mike Tyson. He'll knock you out in the second round a lot of the time, but if the fight has to go on for a long time, he lacks the staying power.