Here is the full list of all 16 Howl achievements.
Defeat the white beast.
Defeat the pack leader.
Enter the Cathedral.
Defeat the origin beast.
Gain every Skull.
Gain all Confidence.
Unlock the last scroll slot.
Unlock level three for any skill.
Unlock level three for all skills.
Use three skills in a single round.
Set fire to three enemies with a single Exploding Shot.
Kill three enemies with a single Piercing Shot.
Kill an enemy while under the effects of Shadow Step.
Kill two enemies using Force.
Kill two enemies with a single Vault.
Kill two enemies with a single Smokebomb.