In the book The Amber Spyglass, what examples can be found regarding the character Will and his perseverance?

The Merriam-Webster dictionary defines “perseverance” as

continued effort to do or achieve something despite difficulties, failure, or opposition.

Considering that definition, I would say that the entire story is loaded with examples of Will’s perseverance.  Will is introduced to readers in book two of the trilogy, and he exists in a world/dimension different from Lyra’s.  He is transported through a portal to Lyra very early on in the book.  From that point forward, Will could persevere in two different ways.  First, he could give continual effort to get back home and leave Lyra to her own problems.  Second, he could choose to support and help Lyra despite the huge risk to himself.  Will chooses the second option.  
When The Amber Spyglass begins, Will and Lyra are not together.  Lyra has been captured, and Will perseveres in his efforts to pursue and rescue her.  He has help along the way, but the help isn’t always as willing to help as Will is.  For example, Will must convince both Balthamos and Baruch to help him rescue Lyra.  That takes a bit of doing because the two angels would rather have Will come with them instead. Will eventually wins out.  
From there, his rescue efforts are met with incredibly high levels of adversity.  Will isn’t only battling humans and animals.  He must combat angels and demons alike.  At one point, Will is attacked by Metatron.  He, Baruch, and Balthamos escape, and Will is left shaken; however, he never loses focus on his ultimate goal of rescuing Lyra.  
Another good example of Will’s perseverance is his first encounter with Mrs. Coulter.  Mrs. Coulter is a very formidable opponent because she is so convincing, and she attempts to convince Will that she is helping Lyra instead of keeping her prisoner.  Will is not convinced and eventually makes his way back to Lyra and saves her.  From there, Will steadfastly stays with Lyra in order to help her beat the evil forces that they are at war with.  His perseverance runs so deep that he is willing to constantly put his life on the line for Lyra.  I believe that this is best evidenced by his willingness to enter the world of the dead with Lyra.  
Will’s perseverance sees him through many difficult encounters, which is why the end of the book is so heartbreaking.  Will and Lyra persevere together and “win,” but they are not allowed to continue existing in the same world.  Once again Will shows great strength of character by willingly going back to his world knowing that it will help Lyra live.