As with most things in this game, this will require a bit of luck, but it's fairly easy if you plan ahead a bit.
The most important thing to keep in mind when choosing a hero for this is their age. You'll want someone in their teens, since it'll take a few battles to reach 50 kills. Attempting this early in the game can be a bit tough since battles don't have a lot of enemies, so I recommend waiting until maybe 100 years in, where you'll get a lot more potential kills per battle. At this point you'll also have some upgrades, and preferably a relic weapon that your chosen hero can use to make them more deadly.
For the battles, try to weaken enemies with the rest of your vanguard and finish them off with the intended killer. Classes with attacks that can damage groups of enemies all at once (mainly Trickshots and Blastcappers, Alchemist variants are no good because of their limited bottles per battle) are good for this. Bring plenty of healing items to keep everyone alive, and try to engage as few enemies as possible at a time. If you're persistent enough, it should only take 3-5 battles for your champion to achieve their life's purpose. If they were using a relic, it'll also have become quite powerful during all this, so that's a nice bonus.