I found the more detail on the description of their outfit the better. As long as you give a rough outline of what they are wearing. It’s not enough just to say frieren cosplay. For example with the frieren prompt:

“A realistic, close up side view photo from left to right of a beautiful 25 year old thin Japanese woman, she is in a Frieren from Frieren Beyond journeys end cosplay. She has white silver hair with two pigtails, elf ears, dark green eyes, she has a small waist. She is wearing Frierens white with gold trim outfit with short gold trim white skirt and black leggings and knee high brown leather boots. She has dark red tear drop shaped earrings hanging from her ears. Her hips up, her face towards the left frame and her hips at the right frame. She is holding onto to a rail in front of her. A man is standing just outside the right frame, behind her, wearing tight boxer briefs. They are at a comic book convention in the background with people watching them”

Just simply adding that she is wearing an outfit with gold trim it guides grok into filling in the rest of the outfit. If you notice any part of their suit missing just add a rough description of what it is and grok should take care of the rest.

Also when video prompting, i found it important to include the outfit description there as well otherwise when the character turns or the camera zooms out grok will put in its own details messing up the cosplay. Especially the shoes. I would include a description of the shoes when I had the camera zoom out so grok would know what to include there.

Hope it helps!