Now my characters are in the level 73 range - and when I try my hands at a level 9 mission.  I get through the first mission with no issues, but then the 2nd half just blows me out of the water.  i.e. An enemy general who starts at full SP and even if I retreat my allies to the back of the map, they are all still in range of his tremendous Grand Cross which smites a member in 2-3 hits, which he can cast 6 times after his move.  So I'm down 2.5 members before I've even really started.

I guess that means I have to equip everyone with 100% Holy protection... but how would I really know that it's necessary before starting the fight?

My characters are too strong for level 8 fights and I gain very little XP from them, but the Level 9 battles are just crazy when you get to part 2.  I gain a lot of XP and levels in part 1, but then I get whooped by some surprize ability, be it the Grand Cross, Fenris' crazy howl or the dark god's ability to negate the first 10 attacks per turn, and then it's all for naught.