I can't successfully beat them! I have the serium that prevents the flayers from using their mind attack in the Drow City but can't drink it. How do I use that potion? It can't be put into a quick slot.

There are two types of potions in the Illithid Tunnels--one of them cannot be drunk, the other one can. Explore the area further and examine everything you find.

Don't use walkthroughs.