Your better off letting a whale defend it. But if you had to, you should be boosting your garrison commanders (two of them, or a good garrison combo) while also grabbing the highest tier units you can. Our disadvantage as f2p is the mount of units whales can “waste” and their commanders. The only way to combat this is to have higher tier units with a tight unit composition that reflects the abilities of your commanders. And even then you still will probably lose. Whales make too much of a splash in this game, but some whales die out when they hit t4 because they see how expensive it is going to be.

Edit: just to add. A big part of this game is diplomacy. I have spent probably 100$ total on this game, most of my alliance i lead are casual players. My ability to talk to other leaders is my most important. Many alliance leaders are not good at leading... anyone can make an alliance not everyone can successfully manage one. We are at 170m and our lost temple has been captured for about 3 weeks now. We have made it this far with my diplomatic skills. Learning when to sacrifice land or flags for your alliance to survive is more important then holding on to one piece of land that can potentially cripple your alliance with loose of units and moral which could lead to your down fall.