I don't think that anything needs to be changed except the time limit or the HP halved, or a little more than that. It is doable but a lot of people don't know what they're doing in there yet. Som and Monaco got it to 1.4 in the last 15 minutes when they were both missing the entire time before that.
Holy does 200% damage to Undead 4, more than it would to Undead 1, so that part of it is fine. You need to have Asperio, Scorpion King cards instead of Orc Skeletons and full crit to break it.
And also afaik Coma doesn't work on it, I tried to coma it for almost the entire time limit (then again I could just be extraordinarily unlucky).