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.

-Agent of Satan, but my duties are largely ceremonial.