Gravity gun expert
Deactivate all 3 scorpion spawners in the canyon
Roasted
Kill at least 50 enemies with flames
Arachnophobia
Kill the spider queen
Impresses
Kill 3 enemies with a single buzzsaw 2 times in a row
Aim bot
Finish a level with accuracy >= 85%
Freedom
Escape the prison
Heartbroken
Finish a level without losing any lives (hearts)
That was close
Finish a level with 0 lives left and health left < 25%
Not a scratch
Finish a level without taking any damage at all
Eat that
Kill 300 enemies with barrels, crates or rocks
Fisticuffs
Don't let core protector boss hit you with his melee attacks even once
Bugged
Kill the bugs without using the flamethrower in the subway level
Worm feeling
Don't let acid worms hit you while you're riding a boat
Dopefish lives
Find dopefish in the subway level
Turn back
Get into dangerous places in the subway level
Die already
Kill the skull helicopter boss
Fresh air
Don't breath gas in the gas chamber trap
Save the animals
Don't let creatures in the jars get destroyed in the laboratory level
Kill the animals
Destroy all the creatures in the jars in the laboratory level
I'm sorry, buddy
Defeat the buddy boss
The end
Kill professor Nathan and escape the facility
Hardcore
Beat the game on hard difficulty
Chicken
Beat the game on easy difficulty before beating it on normal or hard
Explorer
Find the secret level