(Missed something out) *To level up the sentinels the guild you are in must have been registered for Guild Wars. Only the Guild leader and one under can resigter you're guild for Guild Wars.*
Head into your Guild tab at the bottom of the screen then press
and go into the Guild Wars menu. Once in here press
accross to "Sentinals". My sentinels cost me 100 Glory for Attack, 5000 gold for Life, 5000 gold for Armor and 500 souls for Magic to upgrade to level 1. Once you have upgraded each of these then the achievement will pop soon after.
If your requirements are different then just meet the requirements on screen and you will pop the achievement.