Well, if you insist on MBA 13'', then we can't do much though, since you already decided.
If you're still willing to change your choice, DEFINITELY DON'T GET MBP or MBA for gaming

The price and spec is just too OP.
With about RM4k, you can get Acer V Nitro or ILLEGEAR Z5. Both is better than MBA in term of spec itself. There is still other option like Asus ROG G551 or Lenovo Y50
I personally suggest getting Acer or Z5 as Asus service is not good and Y50 screen is not good.
Reason for Acer is best bang of buck. Z5 is more expensive but the best cooling compared to other same spec laptop.
All these laptop comes with i7 quad core, which is better than i5 or i7 dual core. In term of clockspeed and computation. And all these laptops come with GTX860m which is way better than integrated GPU.
To put it short, all these laptops offered way BETTER spec than MBA. In term of RAM, CPU, GPU, Display etc etc.
If your casual game is Facebook game then MBA is your choice, if you are willing to spend for an OP laptop. If your casual game is those demanding game or any game which requires a discrete GPU, then MBA is a no go.
Even Dota 2 need a discrete GPU to play it decently (min 60fps).
P/S: Get MBA if you're using a lot of Apps Store apps and is familiar with its OS, what is so good about Apple product is their ecosystem. If you're not into it, then better get other brand laptop.
Thank you for your feedback.
Assuming if I leave gaming aside ( I used to play the whole collection of Sims and online games ), is it ok to go for MBA 13". The gaming laptop is really heavy and I do burn the graphic card before as the laptop is on almost for 24-7 and the laptop cost is near to 5k which the laptop bought in last 4 yrs.
I try to constraint my use of laptop which is why I pick MBA 13" as the 11" is kinda small. The prob is I do not know which size to take. 256 or 128