Why am i being hit in all 2nd edition IE games with rolls of like 10 regardless of AC?
Sword spiders has terrible THAC0, they still hit you super easily.
I´m in Severed Hand right now and although i have a perfectly balanced party with a tank
of -11 AC i still get hit at 10 rolls by everything.

I´m basically forced into using grease + entangle every single fight while having
everyone using missile weapons.

A lot of the times you can´t avoid being hit or oneshotted because of setups or ranged attackers.

In BG1you could have an AC of -20 -25 vs missle and still be hit at 14 rolls by archers.

Obviously black isle thought this diffuclty was appropiate, your thoughts?

I think you're missing his point; his issue is that the THAC0 of most of the creatures is unreasonably low, so they hit a lot more often than they should. This distorts the game balance, making ranged attacks and movement blockers (such as web) far more powerful than they should be, and non-AC forms of defense like mirror image or stoneskin far more reliable choices.

I'm not familiar with the 2E creature stats, but I am familiar with the 3E ones and I noticed the IWD2 monsters were noticeably boosted.

I don't know how you work that out. He/she was generalising in his post -- I can only interpret what I read -- as is evident in "I´m in Severed Hand right now and although i have a perfectly balanced party with a tank of -11 AC i still get hit at 10 rolls by everything." That's a generalisation, and without specifics/examples my answer is quite to the (his/her) point.

I think that is his complaint, though: in order for that to happen, a simple calculation shows that they must have a Thac0 or at least -1 if a roll of 10 hits AC of -11. This is not, however, actually too excessive: my BG2 melee characters, who have admittedly basically reached the end of the level curve in terms of thac0 and proficiencies, have thac0s of around -10, once weapon proficiency modifiers are applied. To the OP: check your own thac0 in your character sheet, you may be surprised at how good it is.