Bless
Obviously this spell is most effective on creatures which have a large spread between their minimum and maximum damage, or at higher levels, on creatures which you have many of. If you look at the Castle Army for example, Archangels, who always have a base of 50 damage, have no benefit on Basic, and only get +1 on Advanced/Expert, a miniscule increase of 3 1/3%. But Pikemen go from an average of 2 damage to 4 on Adv./Exp., a whopping increase of 100% damage! Other creatures which get large bonuses include Gremlins, Pixies/Sprites, Air/Storm and Water/Ice Elementals, Halflings, Hell Hounds, Troglodytes, and Harpies.