Backasswards

Backasswards is challenge #31. Isaac starts with 8-10 random items, and starts on the Mega Satan boss fight. The goal is to play the floors backward, hence the challenge's namesake. Every floor ascended will cost one Heart Container (if the player has no Red Hearts, it will take a Soul Heart instead), and the oldest item in the inventory. The player starts each floor with a boss fight (except the, which can be immediately completed), and will have to find the "starting room" to ascend a beam of light to the "previous" floor, similar to the effect of.

Difficulty
Starting with a random selection of items, and starting on the Mega Satan boss fight makes this challenge very difficult at first. If Mega Satan and Satan can be defeated, the rest of the challenge becomes significantly easier. However, no bosses need to be defeated to progress- teleporting out of boss rooms can make the challenge much easier.

Strategy

 * Try to get some really powerful few-item synergies. Resetting is a perfectly good way to ensure good items.
 * Since Isaac will have at least 10 Red Heart containers from the start, and items that grant exploding shots (,, and ) combined is game breaking since as long as he gets caught by his explosions, he can heal indefinitely. They also do plentiful damage to enemies and can open closed doors. Isaac also can heal from Mega Satan's rock waves and mini-bosses spawned by him which spawn ignited Bombs, Troll Bombs or Megatroll Bombs.
 * Having and something that can do damage while not moving, such as, , or , can basically guarantee an eventual win. Damaging orbitals are also a big help.
 * is an extremely useful item to receive in this challenge as well.
 * Even without powerful items or synergies, the challenge of Mega Satan (or any other boss) can be skipped by teleporting out of the room.
 * Isaac only needs a starting item that produces a tarot card on pickup, such as or .  has a high chance to produce a card that can teleport Isaac since it spawns 5 random cards., , and  are all viable teleportation cards.
 * also works as a method of teleportation, but can come with the downside of unwanted teleportation.
 * Teleportation through resurrection by means of items like the is possible but dangerous since most resurrection items revive Isaac with very low health.
 * Taking a hit from Mom's Hand brings Isaac to the starting room of a floor.
 * Going up a floor will remove one heart container and one passive item. If Isaac only has one red heart container, something as simple as leaving the floor itself will end the run.
 * For this reason, be wary of taking devil deals at low health.
 * in particular can be a run-ender, as it removes all but one red heart container on pickup. Additionally, starting with this item will make the Mega Satan fight extremely hard due to the large health reduction it causes.
 * Beware of too, as it has a chance to remove heart containers.
 * Soul Hearts and Black Hearts are not affected by this if Isaac has any red heart containers. Thus, is an extremely useful item for this run.
 * Shops spawn as normal above.
 * and generate closed exits to the next floor, so they cannot help Isaac skip the current floor.
 * Items to be removed first are the "first" items picked up. This is the item in the rightmost column, in the lowest row of that column. If powerful items are shown there, it is best to restart.


 * If a Mom's Dressing Table spawns in the starting room, its icon will show up on the map even if the starting room has not been visited yet. This makes finding the passage to the previous floor significantly easier.