Bless for example increases your to hit chance.
Good point. It does not exactly reduce enemy armor class, but effect is same for hit chance.
Unfortunately, I do not have Bless nor Faerie Fire on my characters but I did try using True Strike - it helps a bit, but is cumbersome since it works only on person who used it.
Faerie Fire would be better than True Strike, since like Bless it may be cast by one character and help all others who attack that enemy. And those 3 are only spells I found that directly improve attack roll. I found no spell that reduce enemy AC ( although Faerie Fire has similar effect ).
Practically all good options that I found on spell list ( Bless and Faerie Fire) would require me to use Cleric, which is not a class I like to play. I even have Xyanyde, but again only character suitable for that weapon would be cleric. In the end, maybe I will have to reconsider it and replace one of characters that I like to play with cleric ... but it is questionable if it would be enough to reduce misses, and game design that forces you to use one class to make other classes viable is not exactly the best.
I wonder if wizard can learn Bless or Faerie Fire? In other words, if scrolls of those spells exist somewhere in BG3 ?