Apparently this technique doesn't work anymore. I'll leave it on here just for shits.
There's a technique floating around on how to get high level nodes while minimizing the cost of your losses. I haven't gotten to really test this out as much as I'd like to but it seems to work most of the time at least. What you need is a god that does a good skill that deals damage. Heaven's Lightning seems to work well as it deals damage to each enemy units. Next you need to use one of each troops type of any you can spare. This means one sword, spear, axe, archer, cav, lancer, and axerider. If you don't want to use the higher cost units then using only infantry can work but it takes more time. They shouldn't get killed though, they'll end up returning to your base.
Basically you send army which is made of one of each troop type and send your god. Your god will do damage to the enemies in the high level node and your troops get sent back because you are by far outnumbered. If you keep attacking you'll notice that the power of the resource node starts going down slowly.
What happens is that the each round has a chance for the god to appear. Sending one of each troop type allows the god more chances of appearing and dealing damage. This is because it forces the fight into more and more rounds.