Blasted Elite Skeleton Warriors. I had only one treasure to go and then they kept popping up. I had physical defense at maximum but they still beat me.
Select Custom Battle, Small, Easy, Aggressive AI, 2 trenches, few mines, no barbed wire, only infantry and machineguns allowed and play as Germany. Just keep keep your trench full of machinegunners and watch as the enemy get slaughtered time and time again (in one attack I saw the British sent 9 infantry against my 9 entrenched machinegunners and they were wiped out but only 1 of my machinegunners were killed).
First get your science research to the max. Then keep upgrading reloading speed, shooting accuracy, health protection, health regeneration and money for killed enemies 1 by 1 and in that order. Buy handgun then machine pistol then machine gun then heavy machine gun. Once all of those weapons are bought and all other research is done wait until you have 11 skill points then spend them on plant growing speed all at the same time. Research or plant growth won't happen unless you press down in front of the lab or tree and hold still without firing.
I did it with a level 25 commando. First Blood for primary weapon, Patriot for secondary Ammo Feed skill and Bouncing Betty killstreak. On level 15 of the campaign hang around on a plane by the nose and shoot quickly at the enemy while moving backwards.
please see my reply to user "numble"