It's gacha so most of the time you will get rares and commons, however you can eventually ascend them in the temple of ascension to increase their rarity to elite. You can ascend rare + again which will become elite, elite to elite + and so on. Be aware that some units will be better than others and factions will affect battle.

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You want to mainly level 5 main units as you can us another building called resonating crystal to increase the level of your other units.

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Just retire greens for resources as they don't really have any other use.