Here is the full list of all 21 Skeletal Avenger achievements.
Started New Game Plus.
Kill Sorcerer's Castle boss.
Kill Dark Caverns boss.
Find all trinkets.
Kill Sewer boss.
Kill 8 enemies with a single primary attack hit.
Collect 10 perks for a single avenger.
Kill Dungeon boss.
Complete the tutorial.
Collect 100,000 coins.
Kill 8 enemies with a single skull throw.
Complete fifty quests.
Find all item types.
Complete a quest without taking any damage.
Complete a quest without using your weapon.
Complete a quest using only dash.
Try out every kind of perk.
Have 100 enemies killed by environment hazards.
Kill 50 enemies with deflected projectiles.
Complete a revenge in 150 seconds.
Kill 25 enemies within 10 seconds.