I assume you maxed out both subclasses. You only mentioned sunsinger and I'm pretty sure you need to max out sunsinger [i]and[/i] voidwalker to get mastery.
English
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The Achievement condition is [b][i][u]A[/u][/i][/b] subclass.
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Oh really? My mistake. Not too sure about that then
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Yeah, the achievement reads as follows: "Warlock Mastery - Fully upgrade a Warlock Subclass - 30 Gamer Score" I got my Titan Mastery by maxing Striker, with no progression in Defender.
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Hmm. That's curious. If you're on PS, click on your username on your friends list so it'll update your trophy info (not sure how you do so on xbox) and see if it's there.
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Xbox One, yeah. I just tried doing the equivalent same thing. Didn't work. Thanks for the idea though.