Reach level 30 in Hardcore mode.

This halfway point in leveling can be quite difficult for new players, although the addition of an Easy mode makes this achievement considerably less imposing than the PC alternative. When creating a new character, be sure to tap Y to get the Hardcore prompt, and switch that character to Hardcore mode. In Hardcore mode, if you die, you die permanently and you must start a new character. Be as thorough as possible in your leveling, to gain as many levels as possible prior to progressing to the next area, and always be sure to keep a finger on RS to roll out of combat.

Note that Hardcore must first be unlocked by reaching at least level 10 in the regular game mode.