Here is the full list of all 20 Impaler achievements.
Deliver an offering without pushing it.
Complete all stages and defeat the boss.
Complete the game without spike kills
Jump on top of a flying enemy
Kill 6 enemies during bullet time
Score over 1,000
Score over 5,000
Score over 10,000
Kill 5 enemies using exploding barrels
Slay every type of monster
Overheat your weapon 10 times during a single run
Unlock all weapons
Unlock all upgrades
Score 20,000 or more 5 times
Complete the game without gun kills
Stomp and kill 5 enemies at once