First mission I ran into her I was unlucky and spawned a pod of 3 aliens. There was no way to avoid one dead soldier but I did manage to finish her off (but of course she keeps coming back to life).
Now I'm maybe a month or so into the game and there is a Resistance Ring mission which is described as "Moderate". What the absolute hell is going on here! Here's the scenario:
I have 5 units with only 3 sergeants and 2 corporals. I have shard gun on my ranger but that's the only mag weapon I could afford. Still on basic armor. So here's what happens (simplified to remove some of the other horror going on):
1) Assassin comes out of nowhere (I can't go hunting her down or I will alert pods to my existence) and stuns and nearly kills one unit. The she runs way out of range of all my units.
2) On her next turn she summons 2 units that are more advanced than any of mine AND she casts some shockwave attack which stuns 3 of my units.
I feel like the only way to beat her is to cheese the game. First I have spread my units out and have some sacrificial lamb up front. Then I have to hope that the assassin chooses to run towards my hidden units behind the sacrificial lamb, and especially within range of my reaper. Other than that I don't know how to deal with her.
Unfortunately I don't think there is a way to skip this mission and I don't see any possible way I could have researched and built better armor at this early point in the game. I have been unlucky to only so far get one scientist but even with another one I would still not be able to unlock magnetic weapons and armor and build both a weapon and armor set. But there must be some way of dealing with this so hope some experts can chime in