I found that difficulty is a lot dependant on what combo you use and in which zone you are.
Be warned that Yeek Archimage despite being very powerfull combo is an extremely low health one, meaning that you can be killed extremely easily, and even 1 shot-killed regularly.
So you must invest some point in Constitution to avoid many, many 1 shot-killed situations.
A tactic i have used to unlock Mindslayer with a Yeek Archimage :
An Archimage Yeek is powerfull and is able to kill Subject Z.
But that's not the immediate concern, the primary goal is to avoid Subject Z killing the other yeek, and he will kill him quicker than you can kill Subject Z.
So get the Freeze spell, it is a must to stop the Subject Z
Additionally, you must take Phase door and improve the skill so you can target the helpless yeek and teleport him far behind you, as even frozen the Subject Z can still kill your poor little buddy.
With a good Freeze and a good Phase Door (and other offensive spells), you should unlock Mindslayer easily
Prodigies are awesome (most of them, as some have a questionable usefullness), definitively improved the survivability of my characters late game instead of the festival of frustrating death it was.