the "hints" button on the menu will simplify this, but:
keyboard arrows = movement
f = chain attack
wall cannons will deal low damage
big guys and bullbeetles will deal moderate damage
saws deal lethal damage
invincibility shields will make you temporarily immune to damage
the chain attack won't hurt the final boss, you'll have to pull the levers with chain attacks to make the great cannons shoot, which will actually hurt the boss