America Level 4

This lvl is not that hard and not that much naval.
Problem is you have to defend your base and in the smae time attack aggressivly. First time it was hard but after increase in stats after a whil its easy. So keep on trying and you will succeed.

My starting strategy when my units were not that strong.
1. Build radar at you base innorth east. Agressivly attack with your land units the air strip in the middle. Get it and defend it. But dont only defend it by keeping an unit on it but destroy the defence system in the north since it will continiously attack u otherwise (silo and longrange art unit). Easiest way is to send up your fast and strong tank unit (if needed give air support) and set it next to it since the they cant hit you. Get the tank base besides it since its a target and if needed can be a source of new units (for example long range art) for final blow at the base in south west.
2. In the mean time its a balance between defending your base (its not alloeed to be destroyed) and attacking the naval units, getting their naval base. You can use your land units in the south for 1st defending and lateron attacking in the final stage. I advice you to keep the units on land since attack is stronger. You can use subs against naval units and put your naval units in a way you block passage towardsyour base. Sincemynaval units were alsonot strong, i focussed on my air support. When you get airstripand green resources nearby i build more airsupport, lateron atfirstineeded also. To build units attankbase so try to safeup. A little.
3.when defended well and obtained their navalbase yourland units in the south can land for the final blow. Using air support by carrier on sea (maybe even creating long range navalunits atthenewly obtained naval base in combination with an attack from the north (maybe new long range art has to be deployed) will finish easily the job.

Hopes this worksfor you