First, I turned off WiFi and used Ethernet instead but it didn't help. Then, I used it for hours without charging because I thought there might be the problem with the charger but it wasn't. I used keyboard cover so i thought it also contribute to the heat so i removed it and the heat just decreased a few degrees. I then downloaded smcFanControl app to make the fan run faster even when normal usage but it still didn't help. I heard that reapplying thermal paste would help cool down my macbook dramatically and then I did it but still no luck. I also run Hardware Test but they were all fine. Resetting System Management Control (SMC) didn't help either.
One day I went to a coffee shop and use my Macbook for hours without feeling hot at all. It was hot a bit at the bottom case but the keyboard and palm-rest area are ok. Whenever i use it at my house, it get hot quickly. Then, i noticed that the difference between my house and the coffee shop is temperature. I don't usually use air conditioning so temperature in my room is about 35 degree celsius and it hurt my macbook pro. Then, I turned on air conditioning in my room and used my macbook pro for hours and the keyboard and palm-rest area are cool. They're very comfortable for typing. The temperature was 25 degree celsius. Therefore, Macbook Pro should be used in cold area, i guess. But, using macbook pro with air conditioning on is very expensive and it's not a solution so i kept searching.
A few days later, I started debugging the problem again. I wanted to know if leaving the back case cover off would help so I let stand on its right (see the image below) and remove the base case off without turning off my macbook pro. I waited for about 15 minutes and it cooled down and it was very comfortable to type. This may be because there was enough air flow in to help cool down the logic board faster as the heat came from the CPU. After that i wanted to know if it still help when i let it stand like that but the back case is off and it was still cold so i assumed there was bigger gap between the logic board and the back case cover while it was standing on it's right side and allowed more air flow.
To create a gap between the logic board and the back case cover, i put pieces of paper in the case on the battery and CD ROM. See the image below. Bingo, it did the trick. I call this a workaround, not a solution. I will find anything safer than paper to put in there though paper seems fine to me.
Summary
There are three things I've done:1. Put pieces of paper to make a gap between the back case cover and the logic board so that there is more air flow to cool down the temperature in the case faster. Or you don't need to put the papers but you can remove the optical drive instead. The later is not as very efficient as the former but the overall temperature in the case is better so it won't hurt the logic board much.
2. Use smcFanConrol app. I make the fan spin 4000 rpm. I heard that using smcFanControl will do damage to your fan but i think it's ok for me as my macbook is 4 years old now and i would rather replacing fan than logic board. However, there is no proof yet. In particularly, you can quit the app if you're in the room with air conditioning.
3. The two steps above will help reduce the heat dramatically but you still feel a little bit warm. You should also use PalmGuard to the heat almost completely.
