Here is the full list of all 29 Skully achievements.
Meet Terry
Summon the Strong Form
Summon the Swift Form
Summon the Vault Form
Defeat 100 Water Punks
Defeat 50 Lava Punks
Stare straight at the Camera
Say Hi to Mom
Complete the Tidal Wave Chase without dying
Complete the Volcano Eruption without dying
Complete the Lava Tube Chase without dying
Find the super secret Wanda Figure
Find the super secret Fiona Figure
Find the super secret Brent Figure
Find the super secret Terry Figure
Find 100% of the collectibles in any chapter
Find 100% of the collectibles in 4 chapters
Find 100% of the collectibles in 8 chapters
Complete any chapter without dying
Complete 5 different chapters without dying
Find 100% of the collectibles in 12 chapters
Find 100% of the collectibles in 15 chapters
Throw Skully 60m
Capture Wanda
Have a falling-out with Terry
Defeat Terry & Break the Jar
Complete 10 different chapters without dying
Find 100% of the collectibles in all chapters
Sacrifice yourself to save the world