A literal phalanx is a group of soldiers marching closely together, or a row of bones in a finger or a toe. (There are also several other specific meanings of "phalanx" in other sciences--insects, botany, taxonomy--but the basic idea is the same: a phalanx is a tightly-structured array of things.)
Figuratively, a phalanx is any powerful, tightly-organized group of people or things that reminds you of an orderly, well-armed array of soldiers.
Several ways are accepted by dictionaries.
I prefer "FAY lanks."
Part of speech:
Noun, the countable kind.
(Countable nouns, like "bottle," "piece," and "decision," are words for things that can be broken into exact units. You talk about "a bottle," "three pieces," and "many decisions."
Likewise, talk about one phalanx or multiple phalanges ["fuh LAN jeez"] or, to be more casual, multiple phalanxes.)
None are common, but if you need an adjective, use "phalangian" or "phalangic."
How to use it:
When you want to compare a group of things or people to a group of powerful, tightly-packed soldiers all marching toward a common purpose, call them a phalanx.
Often it's a certain type of people who form a phalanx: a phalanx of assistants, reporters, lobbyists, protestors, photographers, etc.
Sometimes we talk about a phalanx of things: a phalanx of cameras, solar panels, fruit trees, and so on.
You can say that people are working in phalanx: "they opposed us in phalanx," "we're pursuing that goal in phalanx."
examples: With their fiercely glossy ads, Ulta and Sephora persuade us to assemble a phalanx of eye shadow palettes.