Tl;dr

sudo pacman -S dunst
mkdir -p ~/.config/dunst
cp /usr/share/dunst/dunstrc ~/.config/dunst/dunstrc
touch ~/.config/i3/pomodoro-notify.sh
chmod a+x ~/.config/i3/pomodoro-notify.sh

In ~/.config/i3/pomodoro-notify.sh:

SOUND="/usr/share/sounds/freedesktop/stereo/alarm-clock-elapsed.oga"
notify-send Pomodoro "Take a break!" & paplay $SOUND

In ~/.config/i3/config:

exec dunst

References