The Orb of Expansion is an elusive resource in Loop Hero that is required for game progression. The only way in which you can acquire an Orb of Expansion is during a single fight by killing 4 monsters, drop chances increase alongside the number of enemies. Below is the best way in which you can farm the Orb of Expansion as it guarantees five enemies on a tile.
If you place a Vampire Mansion card next to a Village card the village will become a Ransacked village for three turns. This will mean that during those three turns the tile will spawn 4 Ghouls and the Vampire Mansion will spawn a Vampire giving you the five enemies you need to take out at once to have a chance of getting an Orb of Expansion. This means you have three turns to try and get an Orb of Expansion from that tile. Once the tile becomes a Count's Land tile after the third turn the Ghouls will stop spawning.
Make sure before starting the expedition to farm for Orbs of Expansion that you visit the Herbalist Hut in your camp in order to stock up on potions, so you can heal during the multi-enemy battles you are going to encounter. Also check before setting out that you have the Village and Vampire Mansion cards in your deck. The best strategy then is to ignore the bosses and focus on trying to spawn as many high enemy count tiles as possible to increase your chances of an Expansion Orb dropping, and then departing as soon as you have picked one up.
Take a look at our Best Card Combos
guide to see a list of the most effective card combinations you can put together in Loop Hero.