Just practice master strokes with a sword with bernard (I’ll explain why a sword). Firstly in about 2 hours of grinding, if even, of master strokes and spamming combos at Bernard you can level strength, defense, warfare, swordsmanship, and slowly agility. This is at the very start of the game essentially. In no time at all I was 1-2 shotting cumans in full mail/plate armor. I cleared the entire camp during the quest Vipers Nest just because I felt like it, taking on 6-7 enemies at once. Sometimes soldiers would just die from blocking my attacks/me blocking theirs.
Now onto why you train with a sword, this is also a bug. If you train with maces with Bernard you will actually hurt him if using practice weapons. The wooden swords do not harm him at all but if you use the mace you can actually get his health low enough to the point where he will stop the training and promptly ARREST YOU FOR ASSAULTING SOMEONE. Damage shouldn’t even be allowed in fights with practice weapons.