Excellent idea!
The wording and pharse a person uses to convey a message is very important to reach you audience. First it must be relevant to the audience for them to pay attention and read the ad.
Instead of making statements it is good to ask questions that make people think, not just for the witnesses. Such as: Are JW's the only true christians? This will put the WTBS on a no win defense. (If they are then they will insult the audience, if they aren't then what relevance does being a DUB have?)
There are guerrilla warfare tacticts that are very effective in instilling seeds of doubt with those who still have a some thinking ability. I put on my "ricky righteous" attitude and watch the wheels in a persons mind start to move. This is great fun at the ass-embly when you get bored or indignent with the dogma of the WTBS. This is a subject for another time.