Valarion,
I am strongly on the side of those who feel you can’t “fight” a god. However, that doesn’t mean you can’t “defeat” him for the purpose of your story.
The showdown with Nurgle shouldn’t be players wielding weapons against Nurgle’s wound score (be it 500, 777 or 2,000). It should be a philosophical debate, or a religious one. Or a distraction.
Nurgle is the god of death, decay and rebirth. So that is the plane on which he must be challenged.
Threaten Nurgle with a player who can’t be reborn after he commits suicide in Nurgle’s presence….
Present Nurgle with a flower which doesn’t decay.
Present Nurgle with a slave army/zealot army/horde of penitent sinners/army of sisters of battle in chainmail bikini’s which commits ritual suicide after being infected before they can suffer and decay, thereby robbing Nurgle of much of his pleasure…
Engage Nurgle in a debate on the different ways of suffering and how religious fervour alleviates suffering.
In short, think up some role-playing way in which to “defeat” Nurgle (just a temporary defeat) by befuddling him, dazing him, engaging him in a puzzle, etc…..
In game terms, this will both require the players to come up with innovative ways of engaging a god (the GM can help them by having them visit with an assortment of seers, wise men, living saints etc. beforehand to get pointers), and maybe occasional dice rolls using perception, willpower, fellowship, toughness etc. (the not combat stats…).
In the end, a meeting with Nurgle should be epic and well…different. Not just another combat with a supersized Greater Daemon. I am sure the players will enjoy and remember something totally different far more than another boss fight.