It’s not too difficult to achieve -100 relations with a specific lord or lady, and you can start working towards it from the beginning of your campaign by insulting them whenever you get the chance, allying yourself with their enemies, raiding their villages, and any other activity that you can think of which will increase their anger towards you. You should consider which faction you’re likely to be making enemies of in the future of your campaign first, though!

As you continue fighting with the chosen lord or lady, you’ll need to make sure that they’re not captured or executed by the leader of any other army, so you should keep up to date with who their enemies are using the encyclopedia in game, and keep an eye on their movements to see when they might be putting themselves at risk of defeat at the hands of others. In the worst case scenario, the lord or lady will be captured, since the AI is reluctant to execute their enemies in most cases, and usually opts to imprison and then sell them instead.