Medan to Singkil, how to?

As a general rule, if you are in Medan, Go to Mejid Raya (Angkot Mr X) then ask any travel agent

In the particular case of Singkil, contact Mr Dermawan (+62 82166457040). He is the manager of "pulau baniak Taxi" and one of the most reliable person in north Sumatra. As far as I know, the price is 140k rupiah per person

There are also minibuses a bit cheaper (100k) but it does not worth the inconvenience. In case of flood, Dermawan would commission another taxi on the other side of the water while the minibus service will tell you to make the last 80 kilometer on foot.

Here are the options:

1. Charter a whole car (with driver). A car with 7 seats (1 driver + 6 passengers) costs 750'000 IDR. I would recommend to pay maximum 900'000 IDR. Journey takes 8h.
2. Share a taxi. They call taxi ( for that kind of journey) a car with 7 seats (1 driver + 6 passengers). The price is 140'000 IDR (in June 2015). Journey takes around 8h if the driver does his job correctly.
3. Take several public buses. I have not done that myself and have not heard of anyone doing it. But I suppose it's possible. Only thing is that you will probably have to change bus at least 2 times. And the difference in price with a shared taxi is probably very small (30'000 maybe). Journey time is probably minimum 10-11h.

Book your shared or chartered taxi

Contact a tour agency or a hotel in Medan. Contact them by phone, emails are things of another world for many people in Sumatra. Mind that locals are not very used to plan things very much early before the d day.

Best thing is to contact 3 or 4 agencies and bargain as little.

You can also contact Mr Dermawan, he is quite famous in Singkil for organizing journeys & tours for tourists. He seems like a nice person. As pointed out in this question's comments travel.stackexchange.com/q/48622/9395 Regarding Dermawan, the phone number you provided is out of date, this is the correct one: +62 82166457040. Dermawan Email is dmawan_skl76@yahoo.com, He is a reliable man. But in Sumatra, nobody is reliable enough to keep a phone number more than a few months.

Shared vs chartered taxis

Shared taxis are nice, they are modern, have AC (put your gloves on), but they often get overpacked with passengers, bags and even delivery boxes. Yes, those taxis also deliver packages so it can get a bit out of hand. Same thing goes for picking up passengers, been on a Singkil-Medan journey where the driver did an extra 2h drive (in some horrible non tarmac country side roads) to pick up 2 passengers. Sometimes it's simply much better to charter a whole taxi for a little bit extra.