Florianne is a rogue-like opponent who can be fought or arrested depending on your choices. If fought, she is agile, uses stealth, and can deal significant burst damage. She will frequently disappear and reappear, often targeting your squishier party members. Use abilities that reveal stealthed enemies (e.g., Varric's 'Full Draw' upgrade, Solas's 'Fade Cloak' upgrade). Crowd control is effective against her. Keep your tank engaged and focus fire when she is visible. If you successfully gather enough evidence and have high court approval, you can avoid this fight entirely by arresting her.
