Warframe - Hallowed Flame - Mission Four: Endurance (Halloween 2019 Event)
Running the new Hallowed Flame event solo. The endurance mission is definitely a hard one to do solo without a coordinated group. The enemies can scale way faster based on how quickly you kill them, so you can get sortie-level enemies in less than ten minutes. I achieved the required score of 900 kills and then had to extract simply because by that point I just kept getting knocked down (the new infested enemies introduced with Emissary have an extremely high chance of knockdowns), not to mention even if that wasn't the case, I did encounter a Juggernaut as soon as I hit the threshold. Whether or not that was intentional or bad luck, I'm not sure.
I also hunt down all three caches during this run. I recommend you do as well, because there might be some decent stuff in these things, unlike most other caches.