I like to used Seripham with Saint Celestine, and they are usually in reserves to deep strike in later and lay down a few templates. More or less they are kind of there to eat anything that could instant death Saint Celestine and they reroll their 6up invuln save due to a special rule they have so it makes it a bit easier. I only ever taken them in a small group of 5, and gave them a superior with a plasma pistol and a power sword. If they survive a turn you can have them drop a few more templates and assault a unit, getting extra S3 hammer of wraith attacks and hopefully keep Saint Celestine safe. That is how I have been using them recently, and I am most likely not the best source but that is how I have been using them.