i have the same model that u have and bought about 2 months ago,facing same issue. you know what? the power adapter is not powerful enough to charge the laptop while playing graphics and cpu intensive games like pubg,gta 5,witcher 3 etc.
you can undervolt the cpu or turn off intel turbo boost technology,but i dont think that'll gonna work...in my case i tried both under volting cpu and turning off turbo boost...at first i thought the problem was fixed but it didn't.also if i turn off turbo boost then games get lower fps but if i turn on turbo boost then gets higher fps. so i would suggest you to buy a 180 watt power adapter from asus because the cpu and the gpu alone eats 120 watts when at peak usage(core i5 8300h 45 watt+ gtx 1050 ti 75 Watt) so other hardware(motherboard,ram,rgb keyboard,fan,display etc) will need more power which they take from the battery that's why the battery keeps on discharging while playing graphics intensive games