Achievements - Accomplish the indicated achievement to get the corresponding number of Gamerscore points:

A Comfortable Pace (10 points): Clear mission 11 in Human mode.
Easy Does It (10 points): Clear all missions in Human mode.
Half Way There (10 points): Clear mission 11 in Devil Hunter mode.
Done and Done (20 points): Clear all missions in Devil Hunter mode.
The Best of the Rest (20 points): Clear all missions in Human mode with an S ranking.
A Cut Above (30 points): Clear all missions in Devil Hunter Mode with an S ranking.
Smokin'! (10 points): Complete a Stylish Rank S (Smokin'!) combo.
Smokin' Style!! (10 points): Complete a Stylish Rank SS (Smokin' Style!!) combo.
Smokin' Sick Style!!! (10 points): Complete a Stylish Rank SSS (Smokin' Sick Style!!!) combo.
Simply Spectacular (10 points): Complete a mission with an S ranking.
River of Red (10 points): Acquire 10,000 Red Orbs.
Filled with Pride (10 points): Acquire 10,000 Proud Souls.
Rookie Devil Hunter (10 points): Defeat a total of 100 enemies.

Additionally, there are 33 secret achievements, all of which are currently unknown:

Rock and a Hard Place (10 points): Unknown.
Hardly A Simple Task (30 points): Unknown.
Easier Said Than Done (10 points): Unknown.
All Bow Before You (40 points): Unknown.
Step into the Light (10 points): Unknown.
Tonight, We Dine in Hell (10 points): Unknown.
A Stunning Feat (40 points): Unknown.
Never Say Die (50 points): Unknown.
A Throne of Glory (50 points): Unknown.
Nothing Left Unsaid (10 points): Unknown.
The First Circle (10 points): Unknown.
The Second Circle (10 points): Unknown.
The Third Circle (10 points): Unknown.
The Fourth Circle (10 points): Unknown.
The Fifth Circle (10 points): Unknown.
The Sixth Circle (10 points): Unknown.
The Seventh Circle (10 points): Unknown.
The Eight Circle (10 points): Unknown.
The Ninth Circle (10 points): Unknown.
Covered in Blood (40 points): Unknown.
King of the Palace (50 points): Unknown.
Speak of the Devil (20 points): Unknown.
Modus Vivendi (10 points): Unknown.
Bat Out of Hell (10 points): Unknown.
Your Cup Runeth Over (20 points): Unknown.
Red Orb Millionaire (40 points): Unknown.
Brimming with Pride (20 points): Unknown.
Proud Millionaire (40 points): Unknown.
Skilled Devil Hunter (30 points): Unknown.
Legendary Devil Hunter (50 points): Unknown.
Item Collector (50 points): Unknown.
Skill Collector - Nero (50 points): Unknown.
Skill Collector - Dante (50 points): Unknown.