Code                               Result

Gernade, Gun, Copter, Car          Level 2 
Car, Copter, Copter, Car           Level 3 
Gun, Gernade, Gun, Gernade         Level 4 
Gun, Copter, Gun, Car              Level 5 
Gernade, Gernade, Copter, Copter   Level 6 
Gun, Car, Gun, Copter              Level 7 
Car, Car, Gernade, Gun             Level 8 
Gun, Plane, Car, Copter            Level 9 
Tank, Copter, Car, Gun             Level 10 
Mortar, Tank, Copter, Car          Level 11 
Gun, Copter, Tank, Mortar          Level 12 
Gun, Mortar, Gun, Copter           Level 13 
Mortar, Mortar, Gernade, Gun       Level 14 
Copter, Gernade, Car, Gun          Level 15 
Car, Mortar, Gun, Gun              Level 16 
Copter, Jeep, Grenade, Plane       Level 17
Plane, Tank, Mortar, Jeep          Level 18
Gun, Tank, Copter, Jeep            Level 19
Gun, Mortar, Plane, Mortar         Level 20
Mortar, Gun, Grenade, Mortar       Level 21
Plane, Gun, Grenade, Gun           Level 22
Mortar, Plane, Gun, Plane          Level 23
Copter, Jeep, Grenade, Tank        level 24
Gun, Copter, Copter, Jeep          Level 25
Mortar, Copter, Gun, Jeep          Level 26
Plane, Tank, Plane, Machine Gun    Level 27

Thanks to Revolution readers Ryan Stanley and Emma Wood!