There's a huge discrepancy between power level of classes and abilities. Your experience with PotD will vary depending on what you have in your party. For example you can overcome dragon defenses pretty reliably by just using Inspiring Radiance and Painful Interdiction. I had 2 priests and Inspiring radiance stacks and is cast instantly on whole party, then at least one of Interdictions lands a graze, then wizard with an Eldritch Aim has a very good chance of landing Miasma. That gives you +20+20+15+24 on Will attacks. Then you do whatever you want. I's been quite a few years so details are fuzzy, but I remember double Shining Pillar + Cleansing flame is about enough to kill a dragon outright.
Neither of those are Will attacks, though. One of the issues I ran into was that Will attacks were mostly from my Cipher, but my Cipher usually only had enough focus to fire off a few, then she couldn't recover the focus because of the dragons defenses. I would try to farm focus at the beginning of the battle against the blights, though that usually put the Cipher, who was an off-tank, in harms way close to the dragon in order to pull it off, and it relied on some good die rolls. I think once I got to about 90 focus when the blights went down, but that's only 4 attempts at paralyze. Usually I only had enough focus for 2-3 attempts.
I was casting Interdiction at the beginning of battle on everyone, maybe I should have saved it until the dragon at the end as well. The strategy that worked was just saving every single thing I had during the rain blights, trying not to get wiped out, then dumping it all on the dragon at the end hoping that some would stick.
Keep in mind that this is probably with the upscaled PotD dragon. Maybe choosing the upscaling option was a mistake, I got to the dragon in my initial aborted playthrough (also PotD) before White March came out, and don't remember it being this weird.
Actually, I'm looking at the dragon stats now, and it's showing the upscaled Sky Dragon to have higher defenses than both the Adra Dragon and Alpine Dragon.