First things first warlocks in my opinion are the most versatile class but that being said no class can exist without the others while Titans are able to fight up close and personal warlocks can provide support while also dealing damage at a good rate and hunters are able to pick off threats from a distance or silently kill enemies without all three the battle would fall apart but if they all fill their roles correctly then the fight will be won easily.
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Titans are technically the most versatile since they have dedicated Offensive & Defensive subclasses.
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While I agree that Titans are extremely useful I feel that warlocks and Titans are meant to be used in different ways Titans are easily the best at short range but a warlock can be useful at almost any range