For this achievement you'll need to get a lord to make 100 kills during a single battle. This can be done in both the Eye of the Vortex campaign and Mortal Empires campaign.
Many lords are really strong and can do this, including mages like Teclis who have offensive magical capabilities. Kills with such spells do count. A good one for that would be Mazdamuni from the Hexoatl faction. He is a strong mage, and with a dinosaur mount, he can rack up a lot of kills.
Another sure way is to use Tyrion, a High Elf lord. Fully upgraded with all stats put into high own tankiness and weapons can make him very, very strong, being able to go on his own against several groups of strong enemies.
This achievement is obviously easier on lower difficulties, and I suggest getting this one on the lowest difficulty to make sure your lord can go on without getting injured much or getting near death.
You can also make this achievement easier by holding back the rest of the army or by using your other units to distract certain enemy units. You can also choose to attack for example a Vampire army with many zombies, or the Skaven with many Skavenslaves or Clanrats. As they units are of really big groups it's, and their not strong at all, the lord can get hundreds of kills.