Вот статья на вовпедии про боссов
https://wow.gamepedia.com/BossИ там говорится:
Skull bosses
Skull type bosses are the hardest mobs to kill in WoW. Their level is not displayed as a number, but as a death's head sign. When hovering your mouse over such a mob, the tooltip will read "Level ?? (Boss)".
These bosses have a "dynamic" level - when a numeric level is required for some calculation, they are considered to be 3 levels above the player character involved in that calculation. That is, if both a level 60 warrior and a level 65 druid both tank the same skull type boss, the boss will make crushing blows on the warrior as if it was 63, and on the druid as if it was 68. Similarly, the boss will resist their respective powers as a 63- and 68-level creature would. The first skull type boss that a player may fight with in PvE used to be Pyroguard Emberseer in Upper Blackrock Spire, but as of Patch 3.3.0 this mob is now an ordinary level 60 Elite; The Beast in the same dungeon is still a skull boss, however. There are also some skull type bosses that a usual player can not fight with, e. g. Nessy in the Deeprun Tram.
Т.е. как раз и говорится, что босс с черепком всегда на 3 лвла больше игрока. И первый такой босс в игре это как раз элементаль в УБРС.