Here is the full list of all 19 Scoregasm achievements.
You must... ESCAPE!
Finish level 1a without shooting
Get a 9x multiplier
From the start, finish the game visiting only normal levels
Survive for 100 seconds on challenge 15c without shooting
Get a perfect on level 15c
Unlock every level
Complete every challenge level
Get a combo greater than or equal to 10,000
From the start, complete the game visiting only insane levels without dying
Score greater than 250,000,000. Challenge games do not count
Earn all of the combo medals. There's one on each stage!
Find the fig roll
Find the chocolate digestive
Find the bourbon
Find the pink wafer
Find the custard cream
Find the jam ring
Find the £5 note