In Game Pause Menu - LB + RB + X + Y to unlock admin command panel
Put in command God
Press Start then go do to admin command to enter it.
Put in command Giveitemnum 357 1 0 0
Press Start then go do to admin command to enter it.
Put in command Summon cw_mummy_character_bp_relic_boss_c
Press Start then go do to admin command to enter it.
Kill with Super PickAxe
Alternatively use kill command
See this done in video @ 9:55
Achievement Walkthrough