Atlatl Starter
The Atlatl was Rarran's first item, and a consistent one to build around. To scale it, he used a Red Piggles R early on from Piggles until he found a Crook in the Mid Game.
With a powered up Atlatl, you can deal a ton of damage quickly, allowing you to play with its low CD or as a Medium Weapon synergy.
Pyg's Gym + Lifting
On Day 3, Rarran found a Pyg's Gym, which provides another big burst to Atlatl. By finding it early, he was able to put a ton of stacks into it with its "When you buy a Weapon..." effect.
This paired perfectly with the skill, Lifting, which also scales with weapons bought.
Octopus
Next we have the Octopus, the spicy sauce of this build due to its 8x Multicast effect. It drops from the Veteran Octopus PvE fight so it's hard to plan around, but if you get it within the context of a build that's already scaling Weapon damage well, it can be a powerful pivot.
If you have Damage taken care of, next you'll need it to cast often, which is solved by the next item, Windmill.
Windmill
Windmill makes an amazing pairing with Octopus and any other Multicasting items because it will proc with every usage. It's also very effective with items with low CDs such as Atlatl.
Once it gets rolling, you have an incredible engine that really stacks up pressure and rushes down your enemy.