Welcome to a complete-ish walkthrough of the steps I took to kill the dragon.
If I had to sum-up my method with two phrases, they would be: hp regen gear, and low-level character.

1. Create a new level one BR (rogue).

2. Level him up to 10.

3. Now that he is 10, keep an eye out for gear with the hp regen stat, and for the ranged weapons: shuriken and crossbow. Stash any such gear/weapons in your loot stash (which is found in the loot shrine).

4. Stop leveling him when he reaches lvl 20.

5. Now, hopefully, you were fortunate enough to find gear to grant you a total hp regen that's roughly three. And a good long ranged weapon (shuriken > crossbow > all else).

6. Equip the gear, stand on the second to last step in front of the dragon, and spam attack.

7. Congrats. The dragon should be hitting 1s on you entirely, and your hp regen will make you a tank that can afk on the very step you're standing on.