Part of it comes from the RNG affix of the boss. Some people find Shroud + adds insanely hard, others kill him in 35 seconds. For Gorefist, he is very easy to avoid without adds (just walk to his right anytime he Naruto runs at you with his sword). The Spores are there to distract you, but at even level, they should die in 1 Hunting Rifle shot or 3 Repeater Pistol shots. The Spores also pick a location away from you, and if your back is to the wall (or near one) they aren't going to spawn behind you... HOWEVER, when you are in the fight out in the open and turning around a lot, it's very easy to get them from behind.
Of course, I'm not saying adds are good or bad, just that we did tune them to be manageable and they have both logic to prevent certain things from happening (actually spawning behind you if you are near a wall, for example) and hp values to have a specific time-to-kill with every weapon. They are also on a timer, so eventually you can (hopefully) count on when they will show up next and use that time to beast the boss, then get in position for when the spores come.
Let me know if you find any of this to be true or not and I can run some more tests to make sure everything is working as advertised. If it isn't, we can certainly make some changes!
Thanks for the feedback!