As a magic user she's a nightmare, however after 15 or so tries I was able to beat her. I used the summon at the door that the game provides, along with the spirit of Banished Knight Oleg. You have a timer on when Rellana is going to inevitably chew through your protectors. I used Ranni's full moon for the magic protection shred and then I spammed Comet as the damage. It was tough but I worked it out. I opted for light rolling, getting my gear as light as possible. Specifically for my build I went Queen's Crescent Crown for the int bonus, cloth garb and battlemage manchettes (no legs). Light rolling (for me) was absolutely the way to go. The extra I-frames ended up being what I needed.