What's the difference between Guardian Angel I, II, and III?
The varying ranks of Guardian Angel affect not how powerful the effect is, but who it can affect.
Rank 1 = Core Creatures only -- Praetorians, Vestels, and Marksmen.
Rank 2 = Core or Elite Creatures -- Imperial Griffons, Radiant Glory, Sun Crusader
Rank 3 - Core or Elite or Champion Creatures -- any Haven unit.