I went through the first intellect devourer fight and as a DM I think this fight is a bit out of line. The player has two characters who will have around 8-10 HP on the average and the IDs will do enough to kill one of them in two blows. Ranged fire is unreliable, I generally do no more than 3 points of damage with a bow. I beat the fight by running a cleric alongside Blackheart with both using shield of faith and guiding bolt, options that I think surpass what most characters have available at that point in the game.
Or you could avoid them till you have a bigger party and come back later; That what I did. There are others road, the game isn't a corridor. If you played DOS2, you couldn't miss that there are zone that are too difficult / too hight level for you at first, and you have to come back to it later.
