If you don't already have one you don't need to get an exotic, as we are looking for invulnerability here and not damage. You can use the trading post at Marshwatch Haven.
Here's the list of specialization you should be running. The major focus here is to reduce cooldown for Earth and Air attunement.
Click on a trait to view its explaination
When the wurm spawn it will spin twice before starts
Very rarely, it will spin only once before it starts.
Position yourself at this spot. You should be right below rock to the left, and one step to the right of it.
The reference screenshots is captured with low graphics. On higher graphics the internal may look different.
When the wurm is about to spit (animation guide), use an invulnerability skill.
The skill should be pressed closer to the actual spit (usually when the head is going down) to be able to cover all 3, as the skills has very short duration
The wurm may spit back to back, so if you have to use Magnetic Shield casting it beforehand could be useful.
The game will show 3 Invulnerable messages, one per direction if you have blocked husks. If there are 9, you have blocked eggs.