Highly recommend first getting Guts up to Lv. 99 through story progression first, then going through the Endless Eclipse mode.
Once you've completed all 100 layers of Endless Eclipse you'll be rewarded with the Berserker Armor skin for Guts, which is like permanent Berserk mode for him. This is best to use while going through beating BERSERK difficulty. To switch to BERSERK difficulty, press Triangle while selecting the level
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.