Ditto lots of patience and one hair at a time.
If your arms and shoulders can't take unraveling all of that at one sitting, just do one section a day and keep it combed every day.
I know you board but try your best to get out there every day for awhile. The longer days should make that easier
I would save that worst knot for last. The lesser knots will untangle faster for one thing, they will give you plenty of practice for another
If it makes you feel better: When I was a kid, my horses ran on 98i acres that never got bushogged. During the last weeks of school, when I had to study for exams, I had about enough time thru the week to feed.
By the weekend,not only was I staring all those witches knots in the face they were heavily matter with burrs ---- one big flat mat ---- flip one little piece and the entire mane came up
I never once cut a mane over that, I picked the mane out one or two hairs at a time, and doing a big "yippee!" If I could get a small hank of hair unwound at once.
Same went for the tails
That is why I am so anal about keeping our current pastures bushogged. Keeping the ticks and viper snakes away is secondary