We have limited time to devote to this effort. Therefore economy of effort is the order of the day. There are roughly 2.4 billion Christians and 2.2 billion Muslims for whom the Old Testament aka Judaism is critically important. Therefore the most bang for the buck comes from trashing Judaism.

It is not very difficult to shoot down a primitive, genocidal, genital mutilating religion.

The Koran names several of the OT folks including Adam, Abraham and Moses as official prophets. It also name Jesus as a prophet. It does say the Jews got the OT prophets all wrong. Christians say they got it right but Jesus changed everything. Hardly a dime's worth a difference in the attitude of Christians and Muslims towards the OT folks.

Islam is less against against science than Christianity so Adam has to be considered in terms of evolution. (Only one clown in Turkey is trying to push creationism in Islam.) Mythical Adam and Abraham and Moses leaves the Koran without three key prophets. (I think Noah is on their list so no flood and there are four missing.) This undermines several of their key premises. Above all it eliminates the idea that the Jews got it wrong. They now have Mohamed with no teachings to correct.

In regard to polytheism I deal with them by pointing out they were polytheists in the 2nd c. AD with the Temple of Astarte in Jerusalem in that century. No one knows when most of them became polytheists.

I deal with Christians by showing Jesus is a hood ornament on the religion which has nothing in particular to do with what the gospels claim he taught.

One faces the same problem with Christians in the US. Even the full basic 12 years does not teach critical thought. And most college degrees teach only comparing 'experts' rather than independent critical thought.

Thus I suggest an approach which deals with a premise of Islam, that of the OT prophets. It is good on one hand that their religious education includes memorization of the Koran (but little more) so they will "teach" you all you need to know.

The most difficult part of dealing with Islam is that it admits openly that the rules from on high are arbitrary and capricious. Thus arguments to 'making sense' are cut off at the pass. Mohamed was not an idiot.

As fate would have it Islam outside of the middle east does have a more liberal attitude and many consider the rules to have been appropriate to the place and time. The near absolute prohibition against making images of anything is being seen as appropriate to the times of rampant idolatry but no longer necessary. When you consider the majority of Muslims are neither in the geographic Middle East nor in the political Middle East our tendency view Islam as centered in Saudi is way out of date.

Years ago I hung out after work at a bar and grill run by two brothers from Bangladesh. I asked up their drinking beer and was informed the Koran was talking about a date wine in Mohamed's time. Who am I to argue with progress? These guys weren't the brigtest bulbs but if not for the (very modest) accents and their saying so I would not have guessed their religion and had thought they were Hindu. It was a bar and no politics or religion allowed but I am certain there would have been more of a neutral attitude if we had talked about it. 

Point being all one really needs is an openness that does not come so much from education but from simply seeing the promised dire consequences of sin do not in fact occur. After seeing violating mideast customs which are separate do not have dire consequences there is room for discussion.

One needs only start the process and the idea is set loose in the community. In addition to the idea of appropriate rules for the time and place we get appropriate explanations for the time and place. It was easier to say the OT got it wrong than the OT is BS.

It is not much different in the West. Fifty years ago one did not start off saying Moses, David and Solomon were myths. And today one should not start off with saying the people of the OT were invented between the time of Alexander and Pompey. People are not ready. They have to adjust to the same idea in their own way. These things take time.