First off, Erik mentioned pulling in the god's spheres, personality, and history. This is vital.
I would suggest having a couple of victory conditions:
- kill the god
- trap the god
- heal the god
None of these should be easy, but all of them should be do-able. Of the three, trapping the god should probably be the easiest (it doesn't solve the problem, just kicks the can down the road a bit).
Killing the god is pretty straightforward: you build the deity as a very-high-level monster, and each side takes turn whacking the other 'til one of the sides can't whack anymore.
Healing the god is probably the most rewarding (on the theory that, hey, the party has a god's favor).
Never having played in 2e, I can't offer direct mechanical suggestions. However...
Gods have minions
- add some flavor with weak creatures with interesting abilities/effects
- Pathfinder has a very low-level creature with an aura which requires everybody within 30 feet to roll twice on attacks and saves, and take the worse result
- add a sense of progression: as the minions fall, the deity gets weaker
- really sell this: describe how the "bull" is bleeding more, getting angrier, kicking wildly, how the plane itself is becoming more chaotic as the mad god's power is leaking out in ever less-predictable ways ...
I can see swarms of flies (a bit on the nose, with a mad bull), or a herd of cattle protecting one of their own. Or, a bevy of bullfighters, somewhat ironically protecting their quarry.
Gods have flavor
- even in insanity, some of the deity's original portfolio should be evident
- were plants part of his portfolio? maybe he takes great care to avoid stepping on a daffodil growing in the field
- ... or, because he's mad, he randomly charges at a flower when the wind shifts and he can suddenly smell it ...
- the cause of his insanity should be involved
- I picture the bull kicking around with various weapons sticking out of its hide, the metaphysical manifestations of the cause of his madness; addressing the cause on the physical plane could cause the weapons to shrink (and, eventually, fall out) on the metaphysical plane
Gods (and very high level characters) warp reality
Split the encounter: half is on the metaphysical plane where the mad bull is flailing wildly, and half is on a physical plane, where the players are trying to collect the last pieces of the MacGuffin which will allow them to kill/heal/trap the deity. Perhaps they need to free the souls that the deity has trapped (and is tapping for power). Maybe they need to restore and re-consecrate his last temple to give him a connection back to reality (and, thus, un-blinding and un-deafening the god, kickstarting the healing process).
How, exactly, the encounter is split up depends on what the party needs to do to reach their goal. If the needs are straightforward, they could flip back and forth each round: defending against the bull's (ever-weakening) attacks while freeing his trapped souls. If the needs are more time-consuming, spend one round with the bull and one minute or even hour away from it (spend an hour putting pieces of the temple back together, then a round avoiding the pull wildly lashing out against the only living creatures he can detect, not knowing that they're trying to help). Or, somewhere in between (every milestone in the "real" world triggers a round or two in the metaphysical realm).
If a character dies, though, that needn't be the end of their contribution to this encounter: even if the plan is to cure the god, perhaps the dead character gets to go to this deity's flock of dead minions and can start to free them, aiding her companions one last time.
Conclusion
The single most important thing to do is really sell that what the players' characters are doing is awesome: they're directly affecting the deities themselves, and, thus, the cosmology itself.
The next is to pull in all of the threads about this deity: why he's mad, what his portfolio was in sanity, and how it's been twisted in his madness.
The least important part is to get the mechanics exactly right: you can always pull his punches or tweak his AC on the fly if the fight is too easy or too hard. Since this is the capstone, though, this is the time to let a character die (but, have something for the player to do if it happens too early in the night).
Good luck, and have fun!