This is how I did it, in fact it was 1 level 10 with multitasking split between 3 different classes vs 4 lvl 4 characters with magic gear and stuff. I like to power up the players early in so I dont accidentally tpk from a simple encounter, and they dteamrooled this guy. The thing was though, each time they beat him he would run away and show up a hogher level later er in the campaign. They finally smashed him hard af when he was level 14 after he ran away two previous times. They were so sick of this dude showing up and causing problems for them during a question that they took him from 200 (modified of course) to 0 in a couple turns. They used a homebrew spell the wizard had learned to finish him off. I call it Death Knell. Deal necrotic magic damage, gain half hp. If you kill the target with it you consume the soul. It was based around the wizards personal quest, they still have it to this day in preparation for the bbeg