Hi! Thank you for the comments. As promised, I am sharing my short KL visit. It could still be improved but not bad for a first timer, I guess. I hope you find something useful, one way or another.Â
I arrived in KLIA2 at around 10:25 AM. There was quite a long queue in the immigration. I also exchange money (but as expected, airport exchange rate is much lower). I have no choice though, since I need MYR (Malaysian Ringgit) to buy train tickets (MYR55 going to KL Sentral via KLIA Express). It would be better to change a few, then, exchange again in KL Sentral.
I was able to go out of the airport at around 11:20AM. I immediately checked where KLIA Express is located. It's easy to find, I just followed the signage along the way.
I rode the KLIA Express train at 11:35AM. It's around 30 minutes of travel and a train leaves every 15 minutes as what Fred said.
There's an airport train every 15 minutes and it's a 35 minute run + boarding and alighting - That's an hour each way to KL Sentral when you add walking here and there in an out of stations.
When I arrived in KL Sentral, I immediately look for the KTM Komuter train going to Batu Caves. However, the train to Batu Caves will leave at 1:42PM. I used the time to have lunch and roam around NU Mall in KL Sentral. But if you’re not into shopping, 2 hrs is a bit too much as in my case. Maybe there is something else to go around near the KL Sentral or you may opt to take a taxi to Batu Caves if you want. I heard of Central Market but I wasn’t able to go there.Â
  Hopefully, next time. 
Assume 2 hours of travel to any given destination.
The KLIA train is about 30 minutes but you have to get around from there.
Half a day isn't long enough to to anything
He was right.Â
Assume 2 hrs which include waiting time. But I think I was able to do few things which is not bad enough with the time I have.
I arrived in Batu caves at around 2:30PM. I used 1hr here to go up the 272 colorful concrete steps, taking pictures and observing the surrounding. I just don't like the foul smell of the surrounding plus it’s too hot.Â
I also noticed that you can wear whatever you want unlike some of the articles I read. I saw a lot of people in shorts, sleeveless, and slippers. Or maybe that is just for entering the other areas or temple which I did not do.
I left for Bank Negara KTM Station at 3:35PM. From here, I walked going to Dataran Merdeka (Merdeka Square).
If you have time visit website for GOKL free bus in KL you will get MAP and routes details it cover most of Tourist destination.
In KL Sentral there is one bus Stop it is near Hilton hotel its Red Line Go KL bus. Goes to Merdeka , National Mosque etc.
I will try this next time, although, I think there is also traffic especially during rush hours.
I looked and walked around the other places nearby such as Sultan Abdul Samad Building and Masjid Jamek Mosque, then to Jalan Petaling (China Town).
There are also nearby areas like St Mary’s Cathedral, Kuala Lumpur City Gallery and the National Textile Museum but I wasn’t able to go due to the hot weather at this time (around 4:10PM). I’ll also check this next time.
I then rode a taxi from Jalan Petaling to Petronas Twin Tower (Suria KLCC). I paid MYR20 which I think is a bit expensive.Â
I walked up to Raja Chulan Monorail Station, then rode the train going to KL Sentral at 7:00PM. Then headed back to KLIA via KLIA Express Train since my flight is at 9:30PM.
Because trains get cancelled without any warning ie you can be standing on the platform and next 2 trains don't turn up.
Oh, and I experienced this. The train did come but we waited for almost 30 minutes or so I guess.
Just allow extra time if you plan to use the Kommuter train route
Thus, extra time (just in case) is really a must. 
Overall, it was a nice trip though short. I will definitely go back and spend more time. Thank you. 