How do I defeat the earth sage ?

Lvl up your wind element to 5 then go battle him.Seal the 2 monster realms next to the sage after that gather most of his monsters near him then use Wind Explosion till 2 or 3 monsters are left then send monsters in to attack while backing them up with wind spells repeat until victory...

First of all train your wind element to 5.Then go battle the earth sage travel to the middle of the map then go seal the 2 realms that are most close to the earth sage capture the mana crystals and leave you're weakest monster on the seal to close it after go to his location try to put all of his monsters in one place close to him then us fire-2 and wind-1 to kill his monsters repeat this till there are 2 or 3 left send your monsters in to attak while backing them up with wind blades id say you need to get 2 gale hounds and a orkus the gale hounds to attack and the orkus's to seal the realms...

You can also use some other spells that weaken him. I found that effective especially in the second round which is very hard. If you are going up against the second level of him then defeat all other sages first, then unlock the spell that weakens more than one enemy to wind at a time. Do that at least twice and then use a high level wind explosion. Also if you get your monsters up to level 50 just simply control them all to the earth sage. They will do most of the work.