King's Heart: 3 bosses above the musketeer
Dark Heart: Challenge the Dark Element
Thief's Heart = Green Diamond (the first obtained from a quest) + King's Heart + Dark Heart + Bandit's Heart (forgot the name, but it drops in the Mercenary Handbook area). Then go back and find the thief (a bit above the mercenary, with an exclamation mark on their head).
No problem fighting the Mercenary Handbook solo. I reached the Elite level after killing 4000+ enemies.
But in multiplayer, I killed 40,000 and was still at the Intermediate level. Sigh...
For the last one, to enter the Dimension Space, you need the Elite Handbook + Pass.
The Pass drops together with the Silence Ring.
After fighting for a whole day ~~~~~~~~~~~~~
Finally passed! ~!~!