Priests + holy warrior medal?
 

Do only spells cast by the hero contribute to the paladin's holy warrior medal. I'm thinking the priest would be a great way to achieve it much more quickly since he has both healing and bless. I doubt it's true, though, as it sounds too good to be true.

As it is stated in the skill's description, only spells casted by the player counts, unit's spells excluded so it means you have to cast it from your spellbook. In my experience: leave a few enemy ranged units alive so they can't do too much harm and just keep spamming the required spells until you reach round 10 (use awaken dragon to cast twice in a round).

There is an exception to this though. The "Magic Shield" ability is only castable by arch-mages, and is a pretty good way to get that skill moving, since you'll likely shield someone every battle, sometimes more than once.

It is true however if you see the requirement for that achievement 50/150/450 use of the mentioned spells... you better start spamming them from your spellbook too and leave the fights as long as you possibly can without risking the zero loss statistics.