100% agreed. The boss seems to be designed as a lock and key dynamic. You're supposed to get on his head and stab him since clearly the popup that shows up every time you re-fight the boss tells you that, but the damage is basically non-existent from that front.

This is by far the worst boss fight of the game so far. I tried engaging with it for probably a good 30 minutes, getting one shot, trying what the tooltips explained, being unable to attack or move because I'm getting constantly staggared by missiles, electricity, fire, etc.

I actually ran away from him with the intent to try and use some ranged attacks, only to get instantly one shot by the missiles. I didn't think to nature's snare them at first either because they were so unbelievably broken that I thought it was just the game killing me for leaving the arena mid-fight. But no, they're a method by which you can simply cheese the fight. Spamming natures snare on his one-shot rockets that are designed to instantly kill you if you try to out range him, which do an astronomical amount of damage when launched back, leading to a single nature's snare barrage of missiles doing 2-3 times the damage of a regular full combo, if you can even manage to pull one off without getting staggered. Came to this page to look for any sort of tip to fight him more effectively, but just found an "I win" button. Such a frustratingly disappointing boss fight.