Warframe offers a wide variety of enemies both challenging and "challenging", and the most commonly found variant of stronger mobs are the so-called Eximus enemies, which cast buffs on their squadmates and debuffs on players.
Warframe Eximus farming might sound like a bit of a stretch if you've only ever seen the Eximi peppered here and there throughout the Star Chart, but you'll be happy to hear that there is, indeed, a place where they reliably spawn by the dozens.
Where to Farm Eximus Enemies in Warframe?</h2>
If you've completed The Sacrifice questline in Warframe, you'll have unlocked the special daily Sortie missions. These are high-level rotating challenges, each of which comes with a special modifier. From time to time, one of the modifiers will be the 'Eximus Stronghold' one, which will ensure that a lot of Eximi spawn in that particular mission.
However, since this is random, a far more reliable way of farming the Eximus would be to join one of Cephalon Simaris's Elite Sanctuary Onslaught missions. It's not out of the place to take down hundreds of high-ranking Eximus enemies over the course of several waves of Elite Onslaught, though do keep in mind that these missions are rather difficult, too.
If you're not up to the task, the ordinary Sanctuary Onslaught delivers a fair Eximus spawn-rate as well, so give that a whirl if all else fails!