Most people find it easier if you capture their base on the cliff so you can build hornets. That's what I ended up doing. In fact, that's become my favorite strategy, capture anything and everything.
That's my little brother's strategy. He likes to capture every building on the map no matter how long it takes him. Mission takes only 20 minutes if you just wipe everything out? He'll spend 2 hours trying to capture everything.
I beat Croatia myself by taking out the north Nod base first. I wiped it out with a group of rail gun predators. My first attack on the air base failed miserably when a vertigo took out my 5 anti-armour zone trooper squads about 2 seconds after the jumped up the cliff. The next time I sent in the predators and second group of pitbulls to take care of them. The infantry base was just a mop up with riflemen APCs.
I hit my first and only mission so far that I just could not beat on hard. The outback mission for Nod was really giving me a pounding. My base came under attack before the 2:30 timer even started and I was always wiped out before the timer got to 1:30. I gave up and played that mission on easy.