Now if you open the door, if you're unlucky with initiative and the enemy attacks mostly hit, your tank will be killed before they have a chance to do anything.
My conclusion: the enemy's average damage output per attack is too high, compared to the hit points of our party. If it were not so, fights like these would be significantly less dangerous while, if the same logic was applied to the party, those where you are the ambusher would not be as easy. Both I would consider very desirable. I dislike very much how much your success depends on going first.
This also explains why lowering the difficulty does not help if you have problems. The problem is not you hitting the enemy not hard enough, but they hitting you too hard before you have a chance to hit back.
I think part of the problem is if you're not a veteran, and don't really know the 'good' spells, and didn't even realize what some things do (I never realized how insane haste was until Act 3 lol), combined with what you mentioned about having to anticipate fights, combined with some fights almost seeming like puzzles where you're meant to solve it instead of just a normal fight, I can easily see how people can be having difficulties.
you're right, some fights have like 10 enemies, and they're not shy about singling out your party members, so you could be down a quarter of your team before you even take a single action. I had a fight where enemies were casting confusion constantly, and I didn't really understand what it was doing, or how to counter it, and so had to solve that with a reload or two. There are so many different mechanics that if you going into this game without DND knowledge or CRPG experience, it can be a ROUGH time.
