Greed attacks by firing fast-moving sets of 3 blood shots from his mouth and by summoning 2 Hoppers (which can be replaced by Trites if the player has Wrath of the Lamb installed) which also begin to attack Isaac. His shots cause Isaac to lose 3 coins, two will be dropped on the ground and one will be lost permanently. This also applies to any contact damage dealt by him.
Greed is one of the harder sins to fight, as his triple shots are fast and he has no animation or sound to warn you before he attacks. His ability to spawn Hoppers is not a great threat, but they act as shields and can make it harder to dodge his projectiles. A general strategy is to stay far away if the player has enough range, and fire tears at him from afar. This allows the player ample time to dodge his basic attacks and deal with any Hoppers he spawns before they become a threat.
Bear in mind that Greed can fire and immediately turn to face Isaac if he moves far enough. He will still have the firing expression on his face for a second, but will not fire.
When defeated, Greed explodes in a variably big shower of coins, which can include nickels or dimes.
He may also drop the Steam Sale item, thus taking 50% off prices in shops when picked up.
When fighting Greed, try not to pick up the coins if you are hit. Because the coins are on the ground, it is impossible for you to lose them. If you pick them up and are hit, then you will lose even more coins.
Eternal Greed's Behavior
Eternal Greed uses 2 attacks:
Spawns 2 Hoppers or Trites.
Shoots 3 homing shots.
Any damage dealt by Eternal Greed will cause Isaac to drop either coins, bombs or keys.
Greed has a chance of appearing in any shop from the Caves/Catacombs 2 onwards. He will spawn in the Caves/Catacombs 2 shop with a 50% chance and always spawn in the shop after that, unless he spawned in the previous floor, spawned in the Secret Room (which happens from the Depths 1 onwards with a 33% chance) or a different Sin is going to appear in that level.
If you have obtained only The Compass but not the Treasure Map, you are able to spot Greed if he is in the Secret Room, since then there's a miniboss icon that won't reveal a grey room icon in the background even when you get next to it. Once you obtain the Treasure Map (or approach the room with Spelunker Hat or X-Ray Vision), the miniboss icon will disappear and you will see the icon for the Secret Room as usual, yet it will still contain Greed.
Only one of the Seven Deadly Sins can appear on a floor in their own designated room, meaning that if the player has already fought one of the Seven Sins in a room marked on the map, Greed will not appear in the Shop or Secret Room.
An example of the Secret Room visible through the Compass, sure to contain Greed
Sometimes, Greed may fire in a certain direction, but have his face turned towards the opposite direction.
When Greed spawns in a secret room, the player can defeat Greed while leaving the jumpers he spawns alive and leave the secret room. Upon re-entering, greed will be alive again, thus allowing the player to kill Greed again to get more coins/The Steam Sale. The x-ray vision allows the player to re-enter the secret room without using a bomb.
When Greed is killed and the player dies at the same time greed will respawn and the coins will stay where they are, this can be achieved by using 1UP, Dead Cat and the Ankh.