NVIDIA Optimus is a technology that allows an Intel integrated GPU and discrete NVIDIA GPU to be built into and accessed by a laptop—Arch Wiki: Nvidia Optimus
Manjaro comes with required softwares to run Bumblebee (NVIDIA Optimus' implementation for Linux) preinstalled but unfortunately it uses open-source driver Nouveau instead of NVIDIA's proprietary one. (Noveau driver doesn't work and will be deprecated, see this)
So we need to replace it.
sudo pacman -R xf86-video-nouveau
sudo pacman -S linux49-nvidia
Edit /etc/bumblebee/bumblebee.conf, go to line 22 and change it from Driver=nouveau to Driver=nvidia. Changes will take effect after reboot.
To test it, run
optirun -b none nvidia-settings -c :8
If you see NVIDIA's control panel, congratz.