These pertain to the Scream Fortress event released on October 29, 2013.

Achievement	How to unlock
Hat Out of Hell	Get the loot from Skull Island in Hell.
Helltower: Competitive Spirit	Win 142 rounds.
Helltower: Hell on Wheels	Send Redmond/Blutarch to Hell 10 times.
Helltower: Hell's Spells	Get and use a rare spell.
Helltower: Mine Games	Score 17 environmental kills in Hell.
Helltower: Skeleton Coup	Kill 99 skeletons.
Helltower: Spelling Spree	Kill 25 players with spells.
The Mann-tastic Four	Earn 4 Helltower achievements.