Ecco the Dolphin

Kill 10 sharks without killing any other animal in between each kill. This can be done by ramming or with the death sonar.

For this achievement you must kill 10 Sharks in a row.

The earliest this achievement can be obtained is in the level "The Lagoon." Right at the beginning of the level you can swim down and find three sharks within the area. You can kill them with a Dash attack (Default X) or the Death Sonar (Default X then B) but only if you acquired it in the last level titled "The Vents." Now, once you kill the three sharks simply swim to the surface for air and return to the area to find that the sharks have respawned. Repeat this until you have the achievement and remember like the description states, this has to be done in a row without killing anything other than a shark. You can also use this method in the same area for the following achievement.