I meant to post about this earlier but forgot. If you are in a Halls of Reflection group where the DPS is a little low to beat the final gauntlet, you can cheat.
Before you start the final event by talking to Jaina, get the whole party to hug up as far away from Arthas as possible but keeping him in LOS. There is a little ledge protruding over the cliff. You should have the cliff dropping away behind you, a rock wall to your right, a crack in the ground to your left, and you should be looking at Jaina and Arthas. Have your tank run over and start the event before rejoining your hug.
Arthas will walk straight past you and you will be out of range of his Curse of Doom aura. If your positioning is a little out you might get a couple of ticks, but these should be healable. Arthas will chase Jaina up the pathway, while you follow behind him. It's important to note that the mobs will still aggro on you, and if your dps is low then you won't have much (or any!) downtime between waves. Nevertheless you will not have to worry about Arthas catching you up since he will concentrate on Jaina.
Once the gauntlet ends and Arthas gets (spoiler incoming) blasted by the cannons, you can just run through the rockfall to escape.
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