What, no one reads the readme? D:
Well, whatever.
Placid is earned for not killing anything with owls. Throwing owls is fine, just don't hit anything with them. Of course, you're allowed to hit the tall guys, but only if they're holding a beagle.
Heap is earned for not letting any items like parachutes, ropes and meds past the screen, i.e. getting all items until your last beagle.
Skyline is earned for not using any ropes or parachutes, so basically only run and jump.
Placid is the single most important bonus, as it gives 200% of the level's score. Skyline gives 60%, and Heap gives 30%. Together, they give the highest possible bonus of 290% of your level score.
However, if you die or lose a beagle even once, you automatically can't get any bonuses.