Go to somewhere with enemies, but not too many, and fight them. Probably the best way to do it.

If they're evil units, go to lothlorien and chop the mallorn trees. It spawns loth elves if you have negative alignment with them. You can also do something similar in Dorwinion by stealing grapes

Generally you want to go for units with low armor/weapons, easy to gang up on, and maybe don't fight back (easy for evil players). You could also harm your units' targets and let them finish the targets off to make this go faster.

Evil Players: Go to the Shire or Dorwinion, the (hobbit and men) citizens here don't fight back and they're numerous. Pelennor Fields is also a good one since Gondorians spawn here a lot, even when conquest replaces spawns with a pledged faction. If you're looking more for a challenge, Rangers/citizens (who fight back) spawn in the Angle and invade frequently in the Eriador region (especially the Lone-Lands and Angle).

Good Players: Gundabad units have less health (and can have lower armor/weapons) than normal orcs, are numerous in the Eriador region (especially the Lone-Lands), and both Gundabad and Angmar invade quite frequently in this region. You can get overwhelmed if there's too many though since they spawn in large numbers (which is Gundabad's biggest strength). Dunlendings are numerous in Dunland and they have little to no armor with wood to iron weapons. Finally, Near Harad has some citizens that have no armor (but fight back), but be careful killing them yourself since most good factions don't support warcrimes (killing enemy civilians).