For this trophy, you simply need to have successfully accomplished a hit chain of 200 attacks or more without being interrupted. This will probably be capable against bosses, as their health will surely outlast the damage that you can do for that period of hits. Don't struggle too much to try and get this off on its own, as you will, of course, have plenty of opportunity to get this. Magic potions do help out, but only the initial hit; if you use Venom, or Volcano, and you create a status effect on them, that will not add to your combo count. However, using said potions or even Cyclone do come with multiple hits attached to them.