This is a mechanic that is only mentioned during loading screens but if two elemental effects are applied to an enemy it creates a special effect called a fusion. This guide will be going over them and their requirements.
Shock + Decay = Miasma
Miasma is a powerful damage over time and it spreads to nearby enemies. Its essentially Radiation damage from BL3. Miasma isn't very good at destroying shields or armor but will shred regular health within seconds.
Burn + Decay = Explosion
The effect is pretty straight forward, the result is an explosion that deals massive damage to all surrounding enemies. The damage is comparable to landing a lucky strike on an enemy and is usually strong enough to break armor or shields.
Shock + Burn = Manipulation
The effect is that the enemy with the debuff will attack other enemies but from my experience the enemy will just stand there and do nothing until the debuff wears out. The funny thing is that if you are able to keep an enemy manipulated long enough to start attacking their allies, most enemy's are not able to hurt each other.
Manipulation is really useful for the 3rd area since there are a lot of enemies that relentlessly fire projectiles at you.
Implementing Fusions in Your Builds
Crown Prince gains much benefit from elemental fusions because his grenades have a very high chance to apply the decay to enemies. With a fire weapon and a shock weapon you will be able to apply every elemental fusion at your disposal.
Aoi Bai can dual wield weapons so its pretty obvious what to do, just have two different elemental weapons and you can apply whatever fusion you want very quickly. I would say that explosion is probably the best to go for because its instant damage.