If you have a lot of gold saved up then you can level up Griffith higher than the difficulties of the levels, unless you've already been leveling him through Free Mode or Endless Eclipse. This can be done on Easy, anyway, so there's not much to worry about but levels closer to the end of the campaign are more difficult and enemies cause more damage.
Next, there's only 6 large Apostles throughout the game you need to beat (and their levels):
- Wyald (Devil Dogs)
- Zodd (Pishaka Invasion - must kill first, alternatively Nosferatu Zodd, The Beast Swordsman vs. the Black Swordsman)
- Lord of Thieves (The Brand's Guidance)
- The Count (Those Who Have Transcended)
- Grunbeld (The Berserker Armor)
- Ganishka (Pishaka Invasion)
These can be handled in any order and for Zodd, who you face multiple times, you only need to beat him once. To be efficient, you can beat Zodd and Ganishka on Pishaka Invasion (the last level of the game) but Zodd has to be killed first.
PS. Mozgus doesn't count as an Apostle, as he is listed in-game as a Pseudo-Apostle.