The Stalker attacking small parties or soloists makes the most sense from his point of view.
Don't you loving dare try to use lore to rationalize poor game design.
This is not even a case of "get better", this is a case of being assassinated by a random event that you are unable to predict or prevent. I don't care that he's powerful, he can have his fancy stagger-slash and teleportation and whatnot. That's fine, that makes him tough. I care that the player is power
less when fighting him. Throughout the entire game you are utilizing a mixture of gun, melee, and Warframe abilities to fight your way through the enemy forces. What are you supposed to do when the Stalker shows up?
-You can try to melee him, but it's unwise seeing as the North Wind mod doesn't slow him down (through that I am led to assume that the other elemental mods do not apply their effects either), your ground slam won't knock him down, and his regular melee attacks will always knock you down.
-You can try to shoot him, but odds are he's got more shields than you do and can deal more ranged damage than you can one-on-one.
-And you can't use powers, for no adequately explained reason.
So now what? You lure him into a group of enemies, hope they take him out? Smash the windows and hope he suffocates? Is that even considered fighting him?
I can understand wanting to make him a challenge for high level parties, but games have been able to produce higher difficulty for larger/higher level parties since as long as I can remember, so there is absolutely no excuse why the Stalker can't be weakened for parties consisting of lower level players or a single person. Hell, I don't even care about the rare drops he has, I'd be fine with a little checkbox that says "keep this imba pos out of my sight forever".
Also, I love that I just did an alert mission and didn't get any of the rewards.