Something the game doesn’t really tell you is that Rain of Fire stacks. You can cast it as much as you want as often as you want and you won’t lose damage for overlapping.
With that in mind, AoE is pretty much just generating shards as quickly as you can and casting as much Rain of Fire as possible. Start with Rain of Fire and then pretty much just continue your single target/cleave rotation, replacing Chaos Bolt with Rain of Fire. Make sure to Havoc when it’s up and make the most of the Havoc window. If anything is going to be alive for longer than 6-10 seconds, I would Immolate it as well as it will generate more shards for you over time than Incinerate would for the cast time.
EDIT: It’s also important to know when to AoE. Rain of Fire will always be the better choice over Chaos Bolt when fighting 5+ enemies. Chaos Bolt will always be the better choice when fighting 2 or less enemies. Havoc makes 3-5 enemy situations more complicated.
Since Chaos Bolt is duplicated by Havoc, Chaos Bolt is the much better shard spender while Havoc is active and there are less than 5 enemies, as you’re quite literally getting double the normal Chaos Bolt damage for the exact same cost. In those situations, you want to build up to 5 shards before casting Havoc on the highest HP enemy. Then swap to another target and spam Chaos Bolt as much as you can while Havoc is active. Once Havoc ends, you go back to using Chaos Bolt for 2 or less enemies and Rain of Fire for 3+.