Simply play and win PvP matches until you get a total of 900+ points for the week and this will unlock the achievement. Getting 1900 will unlock the PvP tier 1 achievement.
Press Y when looking at the kingdoms and the PvP will be the first icon on the bottom left select this and in the ranked section, it will give you three battle options for opponents. the one on the right will be extremely difficult but will yield higher points but I suggest the middle match as with a good team these battles will be pretty easy.
Upgrade your troops with souls to make battles easier