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.
Us, the first talking brain, I suppose is thought to be your first temp companion (further in the nautiloid ship having it at your side is really useful) that's why it starts with 21 hp and a very powerful attack.
The three spidy brains in the crash site can be defeated by a single player, it requires a bit of strategy because once they arrive close to you they hit hard. So if you have any use enhancing cleric or magic spells (the ones that improve armor class are the better), use the acid grenades that you find in the nautiloid, made use of grease too and acid, and remember that you have to be sure to cover all the area in front of you because the little brainy things are smart enough to avoid the dangerous surface, furthermore use pinning. If you have Shadowhearth and Gale in the party you can use a combo of mage armor and the one that improves your abilities throws.