Option 1 (The recommended one)
1. Install git:
pacman -S git
2. Download, compile and install SRB2 libraries:
git clone https://aur.archlinux.org/srb2-data.git && cd srb2-data && makepkg -si
3. Download, compile and install the SRB2 binary:
git clone https://aur.archlinux.org/srb2.git && cd srb2 && makepkg -si
4. Remove the directories you no longer need:
rm -r srb2 srb2-data
Option 2 (Not recommended)
1. Install yay (skip this step if you have an AUR helper of your own):
sudo pacman -Syyu
sudo pacman -S git
git clone https://aur.archlinux.org/yay.git
cd yay
makepkg -si
2. Install SRB2 from the user repository:
yay -S srb2 srb2-data