This can easily be done during the quest "Spellburst" when you are required to investigate Beggar's Nest.
A portal will spawn not long after you meet Josef. The portal will spawn both Foulspawn and Aberrations; therefore when using this method you will also get the Aberrations achievement.
All you have to do is stand killing the enemies without destroying the portal, so make sure you don't destroy both sides of the portal (one side is fine).
A healing familiar also helps as you shouldn't have to use a single health potion this way.
Hope this helps :)