Something that causes unconsciousness might be better, as you will be provided with a weapon that will capture the soul of the Archbishop. Now if only my poison expert PC were around...
Knock out both Father Taro and the Archbishop, kill the Archbishop while he's helpless, a distraction allows the killer/s to escape.
How's that sound?
EDIT: Oil of Taggit could work. Initial damage 0, secondary Unconsciousness. It's a 1-minute delay, but that might actually work in our favor if we play our cards right.
Now, what the hell is a taggit?
EDIT 2: Anything that causes non-Con damage would work too, since we only need him to be helpless, right?
EDIT 3: Terinav Root and Malyss Root Paste both deal Dex damage and sound like they're made from plants.
EDIT 4: Same with Sasson Juice (BoVD, page 44).