Load into a singleplayer game. Pause the game with the menu button. In the box at the top type in "god" without quotes and press the admin command button. Then type in "summon spiderL_character_bp_c" without quotes and then unpause. There should be a large spider infront of you. Get under it and look up at it then pause the game. In the command box type in "kill" without quotes and unpause. The spider dies and you get an achievement.
**UPDATE**
Pawa7hEbEsT says, "if you do not unlock the achievement, set the console to "united kingdom" and the English language"
Lavindathar says, "Spawn the boss, let it run around for 30 seconds. It doesn't work if you kill it instantly."