It feels like my best strategy to win battles is to just send 5 marines. Anything else just isn't very cost-effective. Then you have to pray to RNG that you get a few attacks in before the aliens do. The quicker you can focus down the aliens and reduce their numbers, the less marines you lose. Losing a marine is only like 50 gold so you aren't too upset, but losing a medic is gold plus 150 crystals which is massive.
Looking forward to more research upgrades, currently the food you need keeps going up and up but there's no real way to increase your food income, meaning you have to grind out more battles. Eventually you're going to run out of bodies to throw into the battles as well because there's currently no way to make your marines stronger.