Yes lyn did no damage but she seemed to have done something because sigurd can't kill her by himself

And same like why do I have to deal with some random OC killing all my faves like I'm here for the real fe characters not some random OCs and then they get sooo many alts

Sameee like this is an fe gacha I do not care about these random OCs but as for thr solution if you are willing to summon for emblem lyn her and sigurd can kill her just with their base kits

You can go to inherit skills and look the skill up then click it and it'll show who has it. I recommend always going for the demotes aka 3 and 4 star units especially since aoe skills are easy to get. You can type the skill in if you don't want to scroll ofc

Ye I understand it's pretty confusing if you're coming back but glitter of light is on base hortensia, mythic loki, and edain while aoe skills are very easy to get on demote units but if you don't have glitter of life you can also try and kill her with 2 range aoe unit before she transforms