Hey so I'm not sure if this was meant to be the way it is but when I found multiple level 20 areas I noticed all the level 20 mobs showed immune effects when I shot them with many different weapons. I am level 8 and have progress in gear finding quite a bit so I don't see any other reason to why I can't kill them. I remember reading somewhere that you can take down these creatures but you may need a lot of players, the amount of people wouldn't change the fact they are immune to weapons would it? Or could this just be a glitch people have yet to realize?
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It's Bungie's way of keeping you out of select areas.