- More precisely, it's most effective on creatures which have large relative proportion of max damage to average damage and low damage but high stack numbers. Think of Gremlins with their min 1, avg 1,5, max 2 damage that becomes fixed 2 (+33% from average 1,5) or 3 on Advanced+ (+100% from avg 1,5). Giants and Titans have larger numerical variance (40-60) but their min-avg-max damage ratio is lower - min 1, avg 1,25, max 1,5, so they only get +20% from normal Bless and 22% from Advanced.
The +1 extra damage bonus from Advanced Water Magic is more beneficial to units that normally deal low damage as it's greater percentage of their base damage, so it's even more useful to low level units with high numbers and large variance. Like anything that has 1-3 or so damage values - Pikemen, Troglodytes, Skeletons, Sprites. Hags & Harpy Hags with 1-4 are perhaps most affected. Air Elementals & Storm Elementals have 2-8, same proportions, but they don't get as dramatic powerup from Adv Bless - it's only 1/8th and not 1/4th of "normal" max damage.