While the solution of "cutting off Dick's head" seemed easy enough, there was the nagging problem of getting through Dick's henchmen who were also Leviathans.
Leviathan, also called the Old Ones, pre-date the creation of humans and angels, as well as the soul itself. They would also pre-date any beings with souls, such as monsters. They are therefore among the first creations of God.
Death describes them as immensely powerful and hungry. Before God went on to create more creatures, however, He felt that the Leviathans threatened to consume and destroy everything "in God's petri dish". For this reason, Death states that God created Purgatory and sealed the Leviathans inside to contain them and their hunger.
In the Supernatural order of things, Leviathans are not just bad-assed, they are the definition of the word.
Leviathans can kill both demons and angels without breaking a sweat. Only an Archangel can give them pause. (And that does not mean the Archangel will assuredly win, only that the Leviathan might hesitate long enough to lose its head...)
Leviathans were basically immortal unless bibbed (forced to eat themselves), poisoned with the Blood of the Fallen or eaten by another Leviathan. They can be temporarily (and this is important, only temporarily inconvenienced by borax, no explanation is given). Fates which would normally kill humans (a car dropped from overhead, for example) only causes them to discorporate into a black goo, and reconstitute themselves within the hour.
Blood of the Fallen: The Word of God states that a Leviathan can be killed by a bone of a righteous mortal soaked in the blood of the fallen. There are three fallen whose blood is required; the first of these is a Fallen Angel (Castiel's blood qualifies). The other two Fallen are The Ruler of Fallen Humanity (meaning Crowley, The King of Hell) and a Father of Fallen Beasts (an Alpha). Once this weapon is assembled, the Leviathan only needs to be given a fatal blow to be killed, but it can result in those nearby being pulled to Purgatory with the Leviathan soul.
- Despite Sam and Dean's impressive record for "ganking" the threats of their Supernatural world, the Leviathan leader Dick Roman was both cunning and duplicitous. He kept a piece of the original Dick Roman and using it, he was even able to clone his appearance making it even harder to figure out which of the Leviathan was the real Dick Roman. Only an angel could tell the difference.
So, chopping off Dick's head seems simple enough, EXCEPT FOR HIS:
superhuman strength, speed, stamina, ability to survive almost any injury,
resistance to almost all mortal or supernatural weapons except for the Blood of the Fallen, which required gathering the resources required to kill him included, fighting or tricking, an Alpha (the best Vampire-kind has to offer), the head of the Demonic Underworld (Crowley) and finding a cooperative fallen Angel,
secured compound filled with security systems, armed guards and replicated Dick Romans (also leviathans),
being the leader of a pack of Darwinistically-determined, super-predators so dangerous even God took them out of the game and gave them their own extra-dimensional prison, should inform you that the task was not as easy as it might sound.