If you got the first base, then take a recovery break. i had a hard time too, till i noticed that i can bring the dead back by click on the Bases when choosing a character xD
and just drive thru´ everything, to get the edelweiss to the snipers. then a mortar and theyre done. :)
and try to use orders, theyre overpowered. 2 orders and a character kann kill everything. ^^