Here is the full list of all 15 Biped achievements.
Complete Training 2P level
Complete Rainy Ruins 2P level
Complete Snowy Peak 2P level
Complete High Treetops 2P level
Complete Sunset Ridge 2P level
Complete Rainy Ruins 2P in 15 mins
Complete Sunset Ridge 2P with lese than 10 death
Complete 10 challenges with harmony greater than 90%
Unlock all the outfits in the store
Complete all the single levels
Complete all the Pro levels
Collect all the stars in all the levels
Complete Dense Forest 2P level
Complete Cactus Vally 2P level
Complete Blue Rapids 2P level