Focus with Mages

So I really like playing mages but I kinda just brain-afk about and don't really actively concentrate on who i want to focus with them. For example, I played Viktor recently and really didn't know who to go for - their frontline consisted of Amumu and Leona, so do I W them to get to the backline, just spam my abilities on ANYONE or do I even try flanking? I get that there's different types of mages but generally, which roles of the enemy team do I focus with what type of mage?
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