Start at Dragon Slayer (526 653)
1] The begining: kill Violetmist Opiate (around 336 635) *respawn time is 5 min*
2] Legendary Rank I: kill Mistwalker Magina (around 238 842) *respawn time is 5 min*
3] Legendary Rank II: kill Gaunob Occulist (around 387 894) *respawn time is 5 min*
4] Legendary Rank III: kill Terranis (around 157 623) *respawn time is 5 min*
5] Legendary Rank IV: kill Ethereal Abomination (around 361 973) *respawn time is 5 min*
6] Legendary Rank V: kill Demonic Dialectic (around 182 326) *respawn time is 5 min*
7] Legendary Rank VI: kill Wayth (around 117 746) *respawn time is 5 min* |you need a magic class for this boss (physical immune)|
8] Legendary Rank VII: kill Snaj, Golden Dodonix (around 637 866) *respawn time is 5 min*
9] Legendary Rank VIII: kill Spider King (around 253 465) *respawn time is 5 min*
10] Legendary Rank IX: kill Death (around 135 559) *respawn time is 5 min*
11] Legendary Rank X: kill Snow Killer (around 523 971) *respawn time is 5 min*
12] Legendary Rank XI: kill Dragon's End (around 295 563) *respawn time is 5 min*
After turning rank XI you'll get teleported to place where Blue Dragon spawns.
You can pick one of two quests at Dragon Slayer Elder and either you kill blue dragon or get rank XI.
Rank XI is needed to make the first song tome, that gives +10% hp, every next song tome gives 10% hp more and requires only dragon slayer mark - the ring you get after killing blue dragon.
PS. If you wanna pick the killing blue dragon quest do it when snow is off. Boss is triggered; you have 60 seconds to kill it.