Use all units possible? Are you insane?! If you do that, everyone would be underlevelled compared to later enemies. They'll slaughter you unless you have the appropiate cards when they initiate the attack.
I have to admit, Durant is only semi useful compared to other units but when he class changes he can take on most units. The downside is that in order to stand up to the enemies, he has to be levelled up throughout the whole game until you get the item. Myself, I prefer using the hunters and whoever I chose as my staff user.
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