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?

Because you have an AC of -11 does not mean that you are unhittable. Your AC is subtracted from the THAC0 of the creature attempting to hit you. Let's take an example of a creature with a THAC0 of 1. The calculation goes:
( THAC0 - AC ), so that becomes ( 1 - (-11) ) == 12. As you can see, the creature needs to roll a 12 or higher from a 20 sided die. You are not invulnerable in any way just because you have low AC.

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.