Monsoon will always start off the battle in his smaller form, before quickly melting into the ground and dropping from the ceiling in his full-size form.
Monsoon makes quick hops towards Isaac, occasionally performing the following attacks:
Spits out a large cluster of projectiles towards Isaac, similar to Monstro's vomit attack but larger and quicker.
Spits out a large projectile into the air which slightly homes towards Isaac while raining down a shower of projectiles alongside 2 Drops falling to the ground.
Summons several Bubbles of varying sizes across the whole room, which appear out of the ground.
Bubbles will drift around the room slowly, bouncing off of walls and being able to block Isaac's tears, popping after being hit by a tear or from touching Isaac.
Melts into the ground and then drops onto Isaac's location 3-5 times in a row, scattering a ring of 8 short range projectiles each time he splashes into the floor.
When he drops from the ceiling for the last time he scatters a larger, randomized ring of projectiles.
Inflates himself to a larger size and slides to the center of the room. Once there, he will release large amounts of projectiles in random directions while slowly deflating.
The rate at which he shoots projectiles increases the smaller he gets, while also releasing projectiles which travel in a circular pattern as he does so.
Once he has deflated back to normal size, he releases a ring of 16 projectiles and the attack ends.
Transforms into a smaller size while launching out a pair of charging Dribbles, each with reduced HP, in a wide V pattern towards Isaac. While in his smaller form, he has an altered set of attacks:
Hops towards Isaac and spits a small cluster of projectiles at the height of each jump towards him each time he does so.
Sinks into the ground and performs the dropping attack but without creating any projectiles.
Eventually, he will melt into the ground and drop from the ceiling in his normal-sized form, scattering a ring of projectiles as he does so.
Monsoon is the boss version of Drops and Dribbles, being one of many bosses based off of the lineage of poop-themed enemies including Dingle, Brownie and Basco.
Much of Monsoon's behavior and design is directly based off of Monstro; however, his ability to change size and split into a smaller form is reminiscent of two other bosses, those being Dingle while Monsoon is in his smaller form, and Brownie while Monsoon is inflated.
There are options in the mod's Dead Sea Scrolls menu of the Fiend Folio MOD to change Monsoon's name:
There are currently four choices — "Monsoon", "Moistro", "Jack the Dripper", or "Jack Moistro the Monsoon Dripper".
While his default name is set to Monsoon, he is referred to as Moistro in game files.
Monsoon's sounds are the same sounds Monstro uses in the original Flash version of The Binding of Isaac.
Gallery
Monsoon after inflating himself.
Monsoon in his smaller form.
Monsoon's appearance in the Scarred Womb.
Drawing of Monsoon in the Bestiary and on Isaac's last will.
Flooded Caves.
Drops falling to the ground.
Bubbles of varying sizes across the whole room, which appear out of the ground.
Dribbles, each with reduced HP, in a wide V pattern towards Isaac. While in his smaller form, he has an altered set of attacks:
Flush! — doing this will also cause him to drop the 
Dripping Badge.
Dingle, 
Brownie and 
Basco.
Monstro; however, his ability to change size and split into a smaller form is reminiscent of two other bosses, those being 
Bosses 