Achievement for picking up the magic book item, which is dropped by the second giant snake enemy in "The Ruins" (Field C).
In addition to the mandatory requirements listed in the other solutions, here's what I can add:
-Cannot use restarts. If the opportunity was missed, back to menu. When restarts are used, the item can be picked up, but no achievement will unlock.
-Target-Lock alone is not enough to kill it. You'll have to guess how much damage to put into the snake before finishing it with target-lock.
-Don't carry the maximum number of bombs or you can't pick up the item.
-To get the item to spawn, "The Ruins" (Field C) has to be the 4TH STAGE you play, otherwise the item will never spawn. This was the detail I overlooked at first.