I try to keep 4 or 5 units on the screen: signalman in a trench with a bar (gets him to high experience for fast airstrikes), an officer (hidden in back of bunker or behind a rock, a sniper (for enemy snipers), and a tank / jeep / bunker with frenchies in upper half of screen to draw fire. Call air support when enemy airstrikes happen, and call arty when enemy tanks show up. Use heroism on any soldier exposed to enemy fire. This strat got me past 90 waves.
 "Epic War 2"){kind=icon}
 "Mud and Blood 2"){kind=icon}