I like many others struggled to find a good guide or video to do this trophy. So I thought to combine a few and found a easy guarantee to doing them every time.
Go to Captain Bernard. What you want to do here is go and train with practice weapons, a short sword and no shield. Now just stab at him over and over until you reach sword level 5. Once there unlock the Fehler: Scarmaker combo.
Now go do the combo training, you need to pass this to unlock combos.
Once unlock, go back into combo training, let him hit you about a bit so you can then do the combo. Instead of doing the one he wants, you are going to do the one you unlocked.
Here's where I found no information to help, so here's my tip.
Do not look at the attack cross thing. What you want to do is hit from lower left, then instantly spam the stab button. If you have done it correctly, you should slap Bernard on the side of his head, this means you have done it right. So again, swipe bottom left, spam stab (literally after the swipe)
You should get around 12-15 combos complete each try you do this.
Hope this helps anyone struggling.
P.S. you can check your current combos done by going into the player menu and looking under combat.