I started it around level 12 and I struggled like crazy. While any level could probably do it, I'd suggest letting your game play on a bit and gaining perks and levels so you can survive a bit better and have more health, etc. I'd suggest about level 20, but get some good armor, good weapons, and stock up on insane amounts of potions and definitely take along a follower.
I agree with all of this. Stock up, Heath is nice (Or Armor, but I prefer Health) potions, Re-enforcements the first time until you know your way around. The first time, I went in with a Master Conjurer, with Heavy Armor, and 2 Dremora Lords. It was Challenging. (No Weapon Skills, at 25, not a good plan.)
The second time, I had a level 5 Steel 2H fighter. Wolf Armor, Skyforge GreatswordS (Superior) and went straight for the Bloodskal Blade. Ran out of potions, quick. Fortunately, I had a couple Scrolls of Guardian Circle, those, and the Wait button saved my life. I was on the next boat back to Skyrim.
The third time was Nightmer, a Dunmer Nightblade/Arcane Assassin. (Unarmored, Conjuration instead of Destruction.) To start, I ran the Telvanni quests, and DB arc all the way to Miraak. By the end of it, I'd built up Light Armor, and became more of a Spellsword, so I respecced Sneak for Destruction (Ironically enough.) Levels 15-27, I cleared the entire island.
Last time, I went with my level 50 Dunmer Battle/Archmage Axeon. Light Armor, Destruction (Cloaks) Drainblood Axe. It was tough, mostly because of the Melee specialty with nothing but Lightning Bolt at range (Augmented Shock, Disintigrate.) I barely leveled at all. Most of my skills were at, or near max, so I transitioned to Conjuration without the crutch of Flaming Familiar, nor Bound Weapons, it was slow going. By the time I got 1 Dremora Lord, the difficult quests were over, and the rest was pretty yawn worthy.
I also loaded my one, and only level 81 to dance with the Ebony Warrior. I finally ended up Frost roading him to death around Morthal (Where I ran into him, and the Cultists) with Critical Charges to stagger him, and knock him down in incriments. Untargeted, his Reflect effect didn't work, it still took forever, but he never got the chance to hit me once I died the first time trying to toe-toe with him. Elbereth, High Elf Spellswordy type, but Unarmored other than Ebonyflesh, and Bound Sword. I tried running her through Solestheim, but having already done it all, and with such a powerful character, it was boring.