Supernatural hunter's guide

Angels

Angels are powerful celestial beings of light created by God to serve as his attendants and messengers, but also as his soldiers and warriors in his war against Lucifer and his demons. They are native to Heaven and are imbued with considerable power. Angels are capable of reproducing with humans while in a vessel, creating a Nephilim. However, if they're missing their grace, the resulting offspring will be human. It is unknown if two angels within vessels could reproduce. Angels are also among the most recurring supernatural beings in the series.Angels are difficult to kill. The methods that are used to hurt or kill demons and humans will not affect them at all. In On the Head of a Pin, Uriel states that the only thing that can kill an angel is another angel, though this has been shown not to be the case as both Dean and Meg have killed angels before. Although there are not many ways known, there are methods to stop or kill an angel. Even archangels, who are extremely powerful and superior to all other angels, possess a few weaknesses, though little which can kill them. 

Weakness
Holy Fire - Encased in a circle of flames created from Holy Oil, the angel is trapped. The trapped angel still can use their powers, possibly aiding them to escape. Touching the flame will kill the vessel and the angel. However, it cannot kill Michael, being only able to banish him. Should the circle be disrupted by some sort of bridge, the angel can cross it and escape unharmed.
Lucifer's Cage - This cell can hold any angel, therefore including cupids, reapers, angels, rit zien, seraphs and archangels, without any internal means of escape. However, Castiel was able to at least reach inside of the Cage to pull Sam out though he called the task "nearly impossible" and was only able to get Sam's body.
Enochian sigil - When Enochian sigils are placed on human rib cages, they prevent any angel/archangel from detecting and finding them. They can also be used to prevent angels from entering a building by painting them on the building's walls. In Torn and Frayed, the sigils on the walls dramatically weakened Castiel's powers. They can also prevent an angel from hearing a conversation of those in a warded room.
Hexbag - Hex bags can used to prevent angels from detecting and finding someone (these cannot hide someone from archangels in vessels).
Brain Penetration - One of Crowley's demons was able to block at least some of Samandriel's powers by placing large metal spikes into his brain. They can also break into the angel's "factory settings" through this method.
Higher Demons/Large numbers of Demons - The highest Demons are able to fight, overpower, harm, banish, and even possess lower level angels, they can't kill them though. Alastair was completely unfazed when punched twice by a lower angel and was able to easily overpower him. A significantly large number of demons (in their smoke form) are able to overpower a group of regular angels as Crowley's army of demons was able to defeat Castiel's guards ("The Man Who Knew Too Much").
Horn of Gabriel Sigil - This symbol is able to attract any angel who is around to the caster.
Angel trap - It's another way to trap angels besides Holy Fire.
Angel Imprisoning Sigil - It could temporarily imprison them. When imprisoned, they can lose their powers temporarily. The sigil also block any sound from inside of the warded room to be heard by any angels in outside of the room.
Alcohol - A whole liquor store's supply of alcohol is sufficient enough to cause a normal angel, cut off from heaven's power, to become intoxicated.
Grace Removal - An angel's grace can be removed by slitting their throat and drawing it into another being or a vial which renders them human and mortal.
Angel Depowering Spell - Capable to depower nearby angels if certain trail along with the sigils.
Weak Vessels - Having a weak vessel seems to reduce an angel's power significantly: while in Nick, Lucifer was stated to not be at full power as he wasn't his true vessel and Hael lacked most of her power while possessing a vessel that was rapidly deteriorating.
Blood Seal - If a person places his or her hand upon this blood seal that is placed upon something then all angels in the immediate area will be banished away. An angel can activate this seal as well. If an angel activates the seal then the angel who activates the seal will not be banished along with the other angels in the area. The blood seal can also be place upon a person's body. If an angel activates the seal this way (carved into its vessel's chest), he or she will be banished along with the other angels.
Exorcism Chant - There is an exorcism chant to expel angels from their human vessels and send them back to heaven. It seems that the chant is not well known as the only entity to use it was Alastair. The Whore of Babylon also knew an enochian chant which causes angels severe pain.
Expelling Angels Spell - Metatron used a spell to banish all angels from Heaven. The steps to complete the spell were to kill a nephilium take its heart, to take a cupid's bow, and take an angel's grace (Metatron took Castiel's). The spell burned off their wings, as well as the wings of any angel on Earth, denying them their ability to teleport, and significantly weakened.
Vessel Expulsion - A vessel possesses the ability to expel the angel possessing them if they so choose depending on their strength of will and willingness to be possessed. However, while the angel is rendered without a vessel, they do not seem to be harmed in any other way.
Eve - A being older than the angels themselves, can suppress their powers through her mere presence alone but it is unclear that if she could kill angels.
Lack of independence - Angels are natural born followers and have little understanding of free will. They do not fully grasp the idea of independence and are created to follow their host no matter what. They put too much faith in their leaders and are somewhat lost without them, as shown after the deaths of Raphael and Castiel, when Heaven descended into anarchy.
Magic - Castiel once stated that if the right spells were cast upon a book, reading it could potentially kill him. Also Rowena was able to simultaneously restrain both Castiel and Crowley and then force Castiel to attack Crowley with her guard-dog spell when she had the Book of the Damned with her.
Weapons of Heaven - Lot's Salt can irreparably dissolve an archangel's vessel, as it did to Raphael's. All of Heaven's weapons acting together can seriously wound, and likely kill, Raphael.
Hand of God - These items contain God's power and it can harm archangel to a certain degree.
The Colt (possibly) - Castiel believed that the Colt was the only thing that could possibly kill Lucifer. Although, a shot to the head failed to kill Lucifer, it's unknown if the Colt is capable of killing regular angels.
Hellhounds (possibly) - Hellhounds might be able to harm or kill angels as upon their approach Castiel ran along with Dean, Sam, Meg and two other Demons.
Anti-Christ -  A cambion, destined to be anti-christ, the offspring of a human and demon, is said to be able to char the entire Host of Heaven, yet it is not tested. Only thing a cambion could do so far was turning a cutt-off angel from heaven into a toy figure.
Destroying