The Core + Immolating Spark
Your starting skill will usually help inform what direction you take your comp since it will always be of a Gold rarity. In this run's case, it was Immolating Spark.
Immolating Spark
So although Rarran's opening item was The Core instead of the Immolating Core, he still chose to go for a Burn build.
Since The Core has the effect of being Charged by what's on the left while Charging what's on its right, it can be versatile and adaptable in its build paths.
Regardless of the direction you take it, you'll want to synergize it with fast-casting and/or Multicast items.
The Core
The Rocket Launcher served as one of the build's main primary sources of Damage and Burn while acting as a Charge activator to the left of The Core with its Multicast.
It was also set to the left of the Duct Tape to also provide defensive value with Duct Tape's adjacent Shielding effect. Throughout the run, Rarran would look for ways to decrease its casting time, such as with Feathers from Loot.
Rocket Launcher
Duct Tape
While all other items Charged The Core, the Heavy Welding Helmet was the only item to the right of The Core.
Since Rarran was able to Enchant it with Heavy, it offered a ton of value being a source of offense, defense, and enemy disruption.
Heavy Welding Helmet
Alongside the Heavy Welding Helmet, additional Slows were provided by Cool LEDs and Heavy GPU. These items have a nice synergy as Cool LEDs automatically casts when the Core is used and the Heavy GPU Hastes The Core.
Last but not least among the active board items is First Aiden, which offers more Haste and adds a bit of Healing which can help against Poison matchups.
Cool LEDs
GPU
First Aiden