What happened with me and my friend is that the game set the light to the level of the highest possible power of any guardian instead of disabling. This means if I am not at my max possible level I am underleveled and because my friend is 100 power less than me he gets insta killed and does not do damage
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For dual destiny specifically, its a known issue [quote]Combatants in the Dual Destiny Exotic mission are at a higher power level than expected.[/quote] https://help.bungie.net/hc/en-us/articles/39007140765204-Destiny-2-Year-8-Known-Issues-and-Vital-Information