If your object is to make someone think you have to guess at what will hit the target.
Without wishing to generalise, "little old ladies" who have been JW's for a few decades will not give one moment's consideration to evolution being possible.
Now ... maybe if they are asked how long they have been a witness and how do they feel about being told that 1975 would bring Armageddon? And then when they trot out "some brothers ran ahead" or somesuch you ask how they feel about the generation that saw 1914 with understanding would "by no means pass away" before Armageddon and yet that generation has passed away.
When they hmmm and haww about that one ask them why that doctrine changed in 1995 and again in 2008 and again in 2010 so why would anyone trust the latest understanding of the generation doctrine if all the others had failed to come true?
That might make her feel uncomfortable and the teenage accomplice would see her struggle and bluster to give a cognisant answer.
You could then turn to the teenager and say JW's and their forerunners have been predicting Armageddon to arrive in 1914, 1925, WW2, 1975, the end of the 20th century and nice ladies like your older friend here have given their best years waiting half a century for this promise and there is still no sign of it coming; please examine your religion before you commit further to it.