I never got to the end but I beat the other Gold Box games. Rudy is right; the games were all difficult, but I found the difficulty tended to ease up in the final third, except for certain key fights, in the previous games.
What kind of parties were you creating? The ideal way to play the Goldbox games was to heavily leverage dual classing. Non humans provided no real benefits (Elves could get 19 dex but in AD&D there was no additional AC bonus over 18) and multiclassing wasn't very useful. But dual classing. . . dual classing was OP.
I may be repeating knowledge you are intimately familiar with, but Dual Class characters basically combined the best of both worlds once class #2 surpassed #1 in level. So you wanted all your spell casters to get a certain number of levels in fighter. It wasn't strictly necessary in Pool of Radiance but it really helped (especially in places like Yarash' pyramid. God that brings back memories), since your casters stayed useful. The right level to dual depended on the game, since the first 3 were each level capped. Basically, you wanted to shoot for the best "extra attacks per turn" level. I want to say it was 5 in PoR (3/2 maybe?). That gave you 5 levels of fighter HP, a good hit bonus (better than a mage or cleric would get for several levels).
Then you wanted to do it again in Azure Bonds. I actually had a system. To best leverage this, when I moved to a new game in the series I dualed my existing pure fighters. As soon as they could carry the party, I remade new pure fighters and ditched the somewhat inferior existing fighter/casters. Anyway, I want to say fighters went to 2/1 at 9 or so. And then to 5/2 at 13 or something (but you couldn't do that until Silver Blades iirc).
Anyway, Dual Class characters were key. You could easily wind up with 4-6 Figther/Casters with great martial prowess and a ton of spells. Everybody had very low ACs, lots of hp, and casters could easily fall back on melee - where they were deadly. Spell casting got less useful as the series went on as more monsters like Beholders started showing up (and weren't there Rakshasa in Silver Blades? Maybe I'm crazy). Still useful, but you found yourself running into more and more situations where spells were stifled by monster abilities. Still, the extra buffing and healing was huge.
You could really take advantage of this if you played through a given game with the same characters more than once. Also, I seem to recall if you got stuck you could simply start over with that party, and be better off (this was helpful in Azure Bonds. While you still lost your starting EQ - but you could stash it on characters in the guild, iirc- it was easier to get through the beginning when you knew where all the resting spots were and had more levels and spells at your disposal).
Were you making characters with max ability scores? That definitely made the game easier (but I was fond of the game giving you that choice; another difficulty modifier, basically. Knights of the Chalice does a variant of this, where you can specify how many 18s will be rolled in character creation).
Man this brings back memories. I hope GOG gets the Gold Box games eventually.