Friendly Fire spends mana to fuel Obsidian Dragon and Burning Blade so that they empower each other; to generate mana we use Mana Orb.
Sun Armor and Turbo-Shelly can heal you and remove debuffs.
Bag of Stones with 2x Stone is useful early on when placed near Hero Sword, Burning Sword or Burning Blade.
You need to find 3x Whetstone by round 5 in order to complete Burning Blade. Finding it later is very hard since your shop will be flooded with higher rarity items.
Oil Lamp ensures your Burning Blade actually hits.