Martial Artist Specialist Guide - SP4
To obtain this SP you have to take the Filled with evil energy quest from Carven in Western Krem.
How to start?
Once you take the quest you will receive a Sparkling Gemstone 
which you will have to take it to Catlotte. She is in the Cat Room, which in turn is located at the bottom of Fir Forest.
Tip: Save a private warp point here to quickly return here for this quest.
When you finish talking to Catlotte you will have to go to talk to Rosebud Witch which is in Secret Crevice, map which is in Shanera Dungeon -2 .
You can get to by going Eastern Krem -> Hidden Path (Bottom left) -> Shanera Dungeon -1.
Once you finish talking to her you will have to go to Shanera Dungeon -3 and hunt Ugly Spearman in order to collect 2 Time-Space Piece (Demon Sanctuary).
Then you will have to go to Shanera Dungeon -4 and hunt Ghastly Spearman until you get 3 Time-Space Piece (Demon Sanctuary).
After finishing with the hunt you will have to talk again with Carven who will send you to Catlotte.
In turn she will give you one 
Time-Space Stone (Demon Sanctuary), which will send you to a quite easy TS.
At the end of it you will have the 2 items needed to be delivered to her, once you give them to her you will obtain the Demon Warrior Specialist Card.
If you want to get another Demon Warrior SP you will have to do the whole quest again.