They are able to exploit It's weaknesses
"It's Weaknesses" from Stephen King wikia:
Despite It seeing Itself as the superior being and stating that Its brain embraces the whole continent, It is far from being almighty. Though It does seem to have significant power over Derry and the town's citizens, It displays several weaknesses which the Losers exploit.
For instance, It clearly underestimates and scorns all of mankind, including the Losers. It is notable in many cases that It leaves an open escape route for the victims and lets them run away. This was seen when the young Ben Hanscom encountered the mummy and when Eddie saw the leper under the porch of 29 Neibolt Street. As a result of this superiority complex, It constantly makes mistakes and does illogical things. When Henry Bowers and his sidekicks chase the Losers into the sewer tunnel system, It attacks Henry's gang instead and turns on the Losers after killing two of Henry's friends. It is also mentioned in the novel that It killed a child named Frederick Cowan by emerging from the toilet, and yet It was unable to finish off the Losers one by one using this same method because It didn't believe that It needed to do so in order to kill them.
It is a psychically sensitive entity, so courage and heart can overcome It, even in its most diabolical forms. Once the Losers are together, their strong shared will and love for each other successfully overpower It and Its fiendish machinations. Their strong faith in their various methods of fighting It eventually lead them to victory. The Losers' assault on 29 Neibolt Street forced It to quickly retreat after being hit by a silver slug (because of the Losers' commonsense solution of using silver against supernatural entities).
The novel also states that when It transforms into a shape, It must obey the laws of that form. This clearly means that It is not invulnerable, and its physical forms can bleed and can be significantly damaged and perhaps even destroyed.
It goes into hibernation for approximately 30 years between killing cycles. During Its hibernation, It may be extremely vulnerable to surprise attacks. However, despite having been defeated for good, it has been heavily implied in other books (such as the Dreamcatcher and Hearts in Atlantis) that It may be still alive. It's natural enemy, The Turtle "Maturin," is mentioned in The Dark Tower series and the character Father Callahan even managed to defend himself from a large group of demons using a cross and the Turtle image, in spite of the latter having "died" during this novel. So, it can be speculated that only a part of Its form was destroyed.
They are guided by Ka to kill IT
It is pretty heavily implied and widely accepted that The Losers' Club are brought together by ka into a ka-tet. This essentially means that they are brought together by destiny for a specific purpose. That purpose is to defeat Pennywise.
"Ka" from Stephen King Wikia:
Ka is the will of Gan, roughly synonymous with destiny or fate.
Ka has been described as "a wheel [whose] only purpose is to turn."
Ka may be considered to be a guide or a destination, though it is not a strict plan. The machinations of ka can, with effort, be obstructed, though more often than not, ka still wins out in the end. Ka is not necessarily a force of good or evil, as it manipulates the powers of both and seems to have no definitive morality of its own.
Further, ka "signifies life-force, consciousness, duty and destiny. In the vulgate, or low speech, it also means a place to which an individual must go."
Individuals drawn together by the will of ka for a purpose are known as ka-tet.
"Ka-tet" from Stephen King Wikia:
Ka-tet is a term in High Speech for a group of people drawn together by ka for a purpose.
The gunslinger Roland Deschain describes ka-tet as being "one from many." Susannah Holmes derives an even simpler definition of the term: it is the notion of family. A tet often consists of several central individuals bound for a purpose, with additional individuals playing transitory roles and specific functions within the group.
Groups that might be considered ka-tets which were never specifically identified as such include:
- It: Bill Denbrough, Mike Hanlon, Ben Hanscom, Richie Tozier, Stan Uris, Eddie Kaspbrak, and Beverly Marsh
- The Stand: Stu Redman, Larry Underwood, Glen Bateman, and Ralph
- Dreamcatcher: Gary Jones, Henry Devlin, Joe Clarendon, and Pete Moore
- Cell: Clayton Riddell, Tom McCourt, Alice Maxwell, Ray Huizenga,
Daniel Hartwick, and Denise Link
- Under the Dome: Dale Barbara, Julia Shumway, Rose Twitchell, Rusty
Everett, and Joe McClatchey
They are protected by the Other (Gan)
While the ideas of Ka and Ka-tet indicate that the group was brought together with the purpose of killing It, as noted in the above definitions, this doesn't necessarily mean there was a plan, and this alone isn't necessarily what gave them power over It.
It is alluded to in the novel that they are given protection by the Other also known as Gan who is essentially the supreme being or God of the Stephen King multiverse.
It is continually surprised by the Losers' victories over it and near the end, It begins to question if It is not as superior as it had once thought. However, It doesn't believe that the individual children are strong enough to defeat it. Only with "the Other" working through them as a group does It believe they can harm it.