Yep... Gimmicky fights.
Cuz an spider failing from 5m takes more damage than being hit in the eye by a heavy crossbow bolt...
So what your saying is you want ToEE, and not BG3. Cool they made that game if you didn't know, it's for 3.5 edition not 5e. Furthermore it seems like you simply have a spider probably I can take care of that. Then there is all that with Firkaag. So they pretty much made him near invincible with all that, and you were fine with that? I could quote you what all is wrong with that many things on one monster but I'm not going to. It's not (kept coming out it' snot) the game we are giving feedback on, or even the edition. Still it is good to be able to refrence things. So here goes.
Stoneskin on spider makes no sense. Immunity to backstab (spiders supposedly can see in many directions. Unsure of this may be untrue.) spider caster haste, spiders can cast magic, uh no. Spider that big having high AC no it's size alone, the inability to wear armor. Sure it's chitin can count as armor, maybe factor in it's dex. Would have to look up it's dex. Then subtract for size vs party. May bump it a couple higher. I'll need to look at it to see. Think it's AC was 13. It already has incredible attack damage (one shotting party members. With it's rage/haste it gets the 2 for 1 deal) Aoe it has that with it's long range poison spit. No buffet but it does have little ones to come to it's aid. No spells except for it's ethereal jaunt, no invisibility detection unless your own it's web. No backstab immunity though it could use it honestly.
In my book, I'd much prefer the spider. I'd be annoyed at the abuse of DM powers that created Firkraag, who appears to be near invincible. Were as the spider makes sense.
On to hp amount you factor in what it doesn't have compared to Firkaag stoneskin, spells, immunities, invunerabilities, aoe to outright kill one party member instead of slowly weakening them with poison, magic resist. Instead it has health, ethereal jaunt, some easily smashed babies, and 1 or 2 mid level underlings. Thinking we need to still give that spider something to beef it up compared to Firkaag. maybe stoneskin. Because if you do the math Firkaag had more hp through his resistances, his immunities, his stone skin then the spider does. More attacks do to his having spells, aoe's, buffet. Then there is fact you can send it tumbling if you break it's web.
I know your love for ToEE seems to have blinded you to those facts. Backstab immunity means no extra damage from your rogue, invisibilty detection shoots down one tactic completely, stone skin increases his armor that you said was already high to begin with. also doesn't it halve bludgeoning/piercing/slashing damage? (unsure figured I'd ask) haste meaning multipule attacks/spells per turn, resist to magic so your mage, is just about useless so factor in any damage he'd be doing into total FirFir hp, and the previously mention sneak attack from your rogue. Then factor in any damage he avoids when he sends you flying across the room, with haste that could well be every round. (ignore this if you can get in close in that turn with him again. or equivelent amount of time.) Which unsure if ToEE is turn based or not. If it isn't just adds irrevelance to it since your moves are more hampered in turn based then other forms.
I counted in that area 1 goblin in that area with no wait guts didn't have 50 hp, that was the half orc warrior that had 50. The drow as well was close. Zhents have around 30 roughly. Again though difference of 3.5 and 5e about the amount of hitpoints they should or should not have. Several goblins had between 8 and 15. So wheres all the other 50's at? I'll look more.
