Really? I think his points are rather bad reasoning. Normally Bosses are immune to CC, so Amanitar's Entangle would be a non-issue if we didn't say "Ok, let's let CC affect the bosses" as a general rule. But then, Ecks has his ability to jump, and it would seem unfair to me to assume that Ecks wouldn't be able to use that ability to get out of the root. I've also never noticed anyone complaining that nature damage didn't work on Amanitar, so ruling out Eck's Spit is a bit unfair as well. It's not really poison, but bile, which makes it acidic, and I don't recall mushrooms being resistant to that.

So maybe Amanitar can get some licks in, but Ecks can jump around, pick up a few mushroom buffs, go into a frenzy and rip up Amanitar with stacked berserks.

The biggest flaw in your reasoning is that Eck is smart enough to go for the buffing mushrooms. As far as we know, Eck isn't an intelligent boss. We can assume this because he has no intelligible quotes or dialect.

Speech is a sure sign of intelligence, and even the Oracles, who aren't exactly geniuses, can speak. Most Gorlocs can't. More then anything, I predict that Amanitar's mushroom poisons would confuse and anger Eck. It'd jump around rather helplessly trying to avoid the mushrooms. Since there are a lot more mushrooms that hurt rather then heal, and Eck probably isn't smart enough to tell the difference, in this chaotic frenzy that Eck is characterized by he's almost gaurenteed to never actually attack Amanitar, instead focusing on the mushrooms since they're the ones killing him.

You're assuming Ecks lacks the cunning to make such decisions, but it seems to me you're giving a huge advantage to Amanitar with his bash and entangle option? Seems a tad unfair to me. If you're going to assume that a boss is using their best available strategy, be fair and apply it to both.

Besides, I don't have to assume anything, since Ecks doesn't need to deliberately pick the healthy mushrooms. He can randomly attack them (though personally, my experience is that Ecks does pick somebody pretty well who to attack with his aggro dropping), and even with his AOE, he can get the buff, so it's not really a valid objection. At the worst? He kills a bunch of the poisonous mushrooms so he doesn't get hurt by them, then springs back to attack Amanitar, and eventually either kills some healthy ones, or goes into a frenzy on his own.

Sorry, but I just don't see Amanitar winning the fight.