Ninja with daggers has the fastest 1hit combo, that's also why you only see Ninja botter in Valley middle, even if they use a different approach with spamming attack packets on the VIDs of the mobs, I can't help you with that sorry.
But I can help you with a far more easy solution, good old Cheat Engine is enough for 3x Ninja dmg on
GF servers.
Freeze
m_dwcurComboIndex to
-1, and
m_fAtkSpd to
2.99 (probably server side checks if you go above 3).
You can find these in the src of mt2 in ActorInstance.h.
As of writing this post these are stable pointers for GF, at least on my machine:
m_fAtkSpd (Float): "metin2client.exe"+0x233DEE4, 0x14, 0x85C, 0x0
m_dwcurComboIndex (Byte): "metin2client.exe"+0x233DEE4, 0x14, 0x770, 0x0
I even recorded a video for you.