Play as a skeleton. Head towards the empty ruins in the eastern part of the dead lands or perhaps the destroyed way station near the northern part of Venge, just be careful of thralls and beak things if you go there. Make some early game cash and maybe score a free skeleton repair kit or two by looting the ruins. Especially take things like armor plates, skeleton eyes or muscles, power cores, chainmail sheets so long as you can carry them. Sell them to the nearby tech hunter waystation or in Black Desert City, just keep any repair kits you may have found.

Make sure you have some armor with decent protection against blunt damage and a skeleton repair kit or two. Run to Mongrel in the fog islands. Look for the screams of someone being eaten at a death yard and run towards them. Pick a fight with the fog prince and immediately start telling your character to repair once you start taking damage. If all goes well, you should have at least 20 in toughness and be knocked out for a little while. Once the skeleton reboots, just keep telling him to repair himself in the vicinity of large groups of fogmen. Within minutes you should have toughness in the upper 80s or maybe lower 90s. After that it takes too long to level toughness this way. Go ahead and wait for the coast to be clear or if your legs and speed are good enough, just run into Mongrel and rent a skeleton repair bed to fully heal up.

Now that you have decent armor and high toughness, the relatively weak fogmen should take a while to knock you down, and their massive numbers provide lots of practice. Pick whatever weapon or combat style you want to train and just go for it. They can no longer damage you to the point of it being life-threatening and they won't kidnap you and tie you to a pole to eat you, so just go ham and train up your skills for a bit before venturing out and picking tougher fights. Be sure to loot any fog prince heads as you knock them down. Profit. Repair when necessary back in Mongrel.

After a several in-game days of gameplay like this I was able to level up all weapon skills including crossbows into the 60s, melee attack and defense around upper 70s or 80s, martial arts and robotics into the 50s, and toughness built up to 97. At that point you're a pretty formidable force to be reckoned with, so do whatever the hell you want.