He didn’t know what degree of power the next attack of the lich king would reach, so preparing a Light Prayers was no mistake.
That series of purple lumps was far more powerful than the blisters that came before. Not only were they extremely corrosive, they also came with terrifying impact force. While consecutively resisting a few dozen thanks to the defensive power of his Divine Snail Shield, he still continuously drew back along with the others, as the effects of the Conjoined Spiritual Boost massively went down. At that rate, the cast for Light Prayers wouldn’t be completed.