I had this same problem, took me HOURS to kill her and I was insistent on using my Adamant warhammer
As I'm sure you realised, by like the 3rd or 5th attempt, you vs dawn and the skeleton brothers at their weakest.
So now we need tactics, you've got the best armour and weapons and maybe something like a strength potion?
We've preset our quick prayer so we've got rapid heal, Incredible reflexes and protect from magic ready to go.
Then we're going to enter the hall, she's going to do her circle blast which you immediately dodge by running to attack her, turn on quick prayer and make sure to use your melee and defence abilities.
When the skeletons are called, hide behind one of the pillars (so Dawn can't hit you) and this will cause the skeletons to form an individual line to attack you, so you won't suffer multiple wounds from enemies you can't attack yet, just a patient line of skeletons, for the future waves, do the exact same thing.
Make sure to pray as needed to recharge your prayer points, it's worth coping a hit or two to do it (But not from the purple circle ball attack, avoid that at all costs).
Using this method, you should get it within an hour no sweat!