Defending convoys from land

You just attack them? What do you mean exactly?

I heard that some could use land units to defend convoys disembarking and killed 20+convoys with 2AC. But I found that you still need quite a number of land unit to defend them. I lost 2units experimenting it

You can look at unit stats and health… your best bet are ranged units as they can freely poop on the enemy while taking no damage.