I feel like I’ve seen a lore entry where a Titan uses a golden gun to take out a knight but I can’t remember is there any lore entries that show this or anything similar
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MyNameIsByf has a video on that. He's the lore master to check. . My understanding is that yes, guardians can use other supers, however they are far more proficient with a particular skill set. Just like how you CAN shoot a basketball, but may be a much more gifted swimmer. Yes, NBA players can swim. However they aren't going to dedicate time to master skills that don't naturally fit their body just to lose out to a Michael Phelps.