How do I defeat the Molduga

Bombs are your friend.

The Molduga detect movement on the sand, so this is the primary mechanic in dealing with them. In each of the Molduga areas, there are rocks or debris that you can use as a safe point. When you stand on one of these, the Molduga will not be able to detect you.

From here, you can throw a bomb onto the sand, and when the Molduga gets close, it will notice the movement of the bomb. At this point, it will move toward the bomb, then jump out to attack. When it does this, you can trigger the bomb to stun the Molduga.

This gives you a window to run in and attack it at will. It will recover, so you can run back to safety, and repeat the process.

Of course, this is a basic method of how to deal with this, there are other methods you can invent to make things more interesting (like bomb arrows for example)

If you've already reclaimed Vah Naboris (Gerudo's divine beast), then by far the easiest way to defeat the Molduga is to just use Urbosa's Fury.

The 3 Urbosa's Fury shots you have are enough to kill the Molduga even without dealing any other damage.

And each Urbosa's Fury shot disables the enemy for a while, so you should be able to kill it in just 1-2 shots.

My way is to set a remote bomb on directly beneath me and use ravoli's gale to fly up the moment the mulduga is besides me and shoot a bunch of bomb arrows in the air down to the molduga.