Question Can I get any hints to implement this army formation movement?

I'm having a though time making formation movement like the gif.
While working, I learned about some sampling algorithms, flocking, boids separation, etc and tried to mixed them with mine. have used a* and navmesh. well i still don't have any idea how to make them properly look good. just wanna know if is there a concept about formation movements i'm missing or i'm just not enough to make them.

 

This is a tough senario, Your using A* as a base pathfinding/AI asset. Theres other assets out there that may suit your project better. Have a look at EmeraldAI, as its what i have used for years, and it supports formations and alot more, with great control.

Personally ive never dealt with this type situation, but EmeraldAI might just be what your looking for...

 

I'm always like 'will the player even notice this'
or will they just come around the corner and blast everything? lol

I think what I would try is to fake the look of it with some kind of built in system.
maybe the particle system using noise and world space collisions to send destinations to the navmesh agents.
maybe some kind of squad or spawn groups.

The Gif almost looks like they just spawn out the back.
Don't notice the flocking right away