Most important is, that you always keep distance between you and such enemies. Also sometimes it's better not to get their aggro, for example in forbidden pit its better not to attack as mage or if you do only a few times so the enemy won't start to attack you. And if needed just run away, speed potions are must have for Ironman mages, I had to flee very often, nothing to be ashamed of as long as you survive.
Exactly. IM is for when you want the experience of wanting to have to take constant care with your actions. And having some common sense to now when to run, fall back, live to fight another day. It is not supposed to be getting in heads on guns blazing. It requires carefully planning and patience.
If one does not want to have to play it with the necessary patience and just wants to have casual fun but that it still be challenging, play on Hard.
If you're on IM and knows you'll be facing stun and paralysis, prepare accordingly. First, carefully push the companion in front of you before the fight begins, or maneuver to get under their cover. In the example of the Undermother, dont just casually go challenge her to a duel to death. Carefully push your companion and preferably a summon and a merc to have them lined up between you and her on a distance that you can initiate dialog, so that when the fight breaks, your gang will be already on top of her. Is that troublesome to set up perfectly? Yeah, it will take a few seconds of your time, and most likely require you to move back and forth a few times before it is set perfectly but hey, that's what it means to play IM, it means having attention to detail and truly strive to have the best possible outcomes.
Other than that, plan the needed resistances for both def and attack, plan for required potions and scrolls, and stack a plan A, B and C (have summon scrolls, resistance pots, health, mana, speed and sometimes even invisibility, Hero pots and perhaps even a Tragic Hero pot followed by a heal before the fight begins and things like that)
EDIT: first time I fought the unweakened spider momma she didnt hit me. I saw your video brother, and it was all fine, just one tiny mistake that obviously can happen to anyone. But what I do differently is that I NEVER stand on the side of my gang. My Mage was Elementalist first time, so all summons were weak scroll summons, more distractions than anything, but my thing is, I always position myself to keep BEHIND the gang at all times, no matter what. If the critter moves, I stop attacking all togheter and turn around, until Im firmly positioned undoubtedly behind my precious Griss. I never stand on one of the sides. That makes if far harder to reach me, since the critter has to put Griss to sleep, to which I just run and wake her up, or the critter has to ignore Griss and turn around her, which gives me enough time to run.