It is possible to gain in each level a medal. To reach this goal, you have to kill a number of enemies. The numbers of enemies are:
Corneria: 150
Fortuna: 50
Sector X: 150
Meteo: 200
Aquas: 150
Titania: 150
Area 6: 300
Sector Y: 150
Solar: 100
MacBeth: 150
Venom 1: 200
Katina: 150
Zoness: 250
Sector Z: 100
Venom 2: 200