Delirium: What it is; Tips and Tricks

[Jaquooob]

Ok let's talk about everyone's best friend Delirium.
Delirum in essence is a white, gooey, living manifestation of Isaac's insanity (to isaac that is). It is the final boss in the Binding of Isaac Afterbith+ to give the true ending of the game. It is known to be one of the hardest bosses because of its unpredicable patterns and sometimes glitchiness but has one of the coolest soundtracks (in my opinion) that a boss could have. Delirium has the capability to tansform into any boss in the game. However, MOST OF THE TIME, Delirium mainly transforms into bosses that the player has fought within the current runthrough (regardless if they are from a boss room or not). Delirum is capable of teleporting and moving at phenominal speed. After you see him in one spot, he's probably already been in 4 other places in the room.
The genral idea for any boss in the game really is to beat them as quickly as possible. This boss is one of those cases where it can be VERY troublesome if you leave it around. Now even though having a long fight with this monstrosity may be fun, there is a huge risk factor to it. Delirium starts of at somewhat normal paced for a boss, but gradually will become faster and faster as the tiring fight progresses. During it's last 1/3 of health, Delirium can virtually become uncontrollable and make it feel like an entirely different fight. So beat this beast as quickly as possible if able (however Delirium is immune to the chaos card, sorry).
Items that help greatly are items that heal with kills such as the Leech or Charm of the Vampire, as when Delirium transforms into a boss that can spawn enemies, it is good to farm health off of them. This method can also be used to spawn allies from the infestation perks for your own army of Isaac.
To my surprise, Guppy's paw can be one of the most useful items for this boss and has saved me countless times. When a player has a ton of health containers but no way of regaining health, the player can just pop a paw and gain three soul hearts to continue the assault.
Copies of Delerium have also been known to show up such as 2 Larry Jr.'s or Mama Gurdy and a Gurgling. When enough time passes, the copies will go away but it is good to pay attention to them to at least avoid them. Try to mainly focus on dodging at that point unless you are confident you can kill the copy with ease. The only copy that does not go away is The Lamb's body (if it transforms into The Lamb) but can still be used for the health or ally method as it constantly spawns flies.
Pyromaniac more so than Host Hat are excellent methods against Delirium's green explosive shots and with Pyromaniac can heal the player so there is no need to avoid Delirium's blasts.
However, the most game breaking combination is the Knawed Leaf combined with items that surround the player and damage enemies. There really is no need for much explanation for this method as of course the player is invincible with it and while simply standing still, Delirium will run into the player and any damage surrounding items like Cube of Meat or Forever Alone will damage it. This also helps deal with the Delirium glitches since the player is invincible, they really won't have any effect. This may be EXTREMELY time consuming but if ever forunate enough to have such a combo, just sit back and watch the fireworks.
Hope this helps you guys, feel free to comment any info you wish to share or questions :)

[Fracastador]

I find that slow effects are the 2nd best way to handle delirium, (best way is huge damage, because his non-transformed form does not scale damage).
While slowed his shot range decreases and his shot patterns aren't too bad to avoid. Slow effects also stop him from using his blisteringly fast charge attacks, and give you more time to react to his teleports too. Broken modem gives you most of the same advantages as slow effects, though it makes his moves harder to predict.

I don't like the idea of gnawed leaf here, because as you say, that would take a very long time.

[Jaquooob]

I agree. Thank you for mentioning that. Slowness has provided some decent assistance during the fight for me as well. Definitely comes most in handy during the later fight during its stupid speeds. Also, one more thing about the Knawed Leaf idea. Yes its time consuming and kind of a broken method that might ruin the fight experience, so I don't really enjoy using it. HOWEVER, for playing as the Lost, it is the best method for that character because it combats Delirium's glitches such as the Mom's leg damage glitch which often ruins most people's run. That's at least my general consensus.
Thank you very much for sharing

[kone]

Algiz + Tarot Cloth/Blank Card = 1 minute of invulnerability
Polaroid helps too
Only time Plan C is great, but needs Lazarus Rags or else Delirium respawns