Ah yes, ye old "Canon say so so this author's fanfic is bad" the argument. Fencer has stated that in his version of Harry Potter guns will break shields. Ergo, guns break shields. End of story. You cannot have things both ways guys. If you're here to read a fanfic, read the damn fanfic without nitpicking about the lore.
I only started reading the comments after the first post on OPs thoughts on Guns and Shield magic, but it seems like they aren't calling the fic bad. They are saying OP is blatantly wrong about the Shield charm according to various 'canonical' sources.
If the OP wants to ignore those, fine.
Plenty of other Potterfics ignore them too.
Granted, most of those stories are on Fanfiction . net. Not a lot of good reader feedback on that site.
Regardless.
The Shield spell according to how various people used it in the books, Harry, Hermione, the Twins, can be used a few ways.
Enchantment onto an item like a hat, for a constant shield.
Laying it down as a standalone spell you can walk away from, like Hermione casting it on the entrance to the Tent in deathly hallows.
Finally, the various shapes to defend yourself.
That last one is a 'sustained' spell.
Meaning you have to continuously hold the spell.
If Taylor wants to carry a gun, let her.
If someone gets a shield up before she can shoot them, well she has bugs. Have them bite, swarm or anything that disrupts their concentration on the spell and THEN she can shoot them.
I mean it's not like Taylor isn't familiar with Parahumans with Shields.
She has fought New Wave a few times.
After all, only the Bubble version of the Shield spell would prevent her from sneaking bugs on them.
And that version seems a little advanced for all but the best fighters among magicals.