This is a pretty frequently asked question, but I'll answer it anyway.

First off, Maelstrom is getting to level 100, as said in its description. That might sound super easy, until you realize it's 100*99/2=4950 skills to proficient.

Then we have Tesla. It is officially unknown, but the community has figured out that it's 10 million energy points. It's even been confirmed by a former KA team member.

As for Atlas and Artemis, the source I previously mentioned said that Atlas keeps on moving out of reach in number of mastered skills, while Artemis is out of reach. That was in 2018, I suspect. There have been rumors floating around that Atlas is 5000 mastered skills and Artemis 7500, but I can't say for certain; I have found no confirmation from KA.