The Alchemaster, like most other bosses, does not spawn on its own and requires the player to summon it with a Ancient Mosaic at Night.
The Alchemaster will fly in the air, throwing Poison and Fire flasks, giving their respective debuffs if it hits the player. He flies relatively slowly and can clip through walls. Occasionally, he will spawn Plague Souls that act as homing missiles that can also clip through walls and will continue to target the player until killed.
The Alchemaster has no new movesets, but becomes much deadlier. His flasks deal around 200 damage, and he now spawns up to 10 Plague Souls at once, rather than 5-6. Plague Souls also have double their regular health and damage
You should only fight the Alchemaster if you focus on alchemical weapons. If not, it is recommended you skip this boss.
The crafting of the Ancient Mosaic is very difficult to do, which is time-consuming.
The Alchemaster is easily exploited with the use of a platform based sky bridge as its flasks will harmlessly pass through the platform, thus negating the 200 damage flasks of expert mode.
It should also be noted that when summoned the Alchemaster spawns at ground level and will have to fly slowly up to you if you are on a sky bridge, giving you ample time to attack downwards at it and remove a large chunk of its health before the fight even begins.