Like with The Mighty Octostomp boss battle, once you arrive on the central platform, spend some time inking the platform which will help you a lot during the first round. You can also use Seekers for the purpose.
Splatoon Dreaded Octonozzle Boss Tips and Strategy
As soon as the battle commences, go on the offence but remember to dodge the large purple ball of ink erupting from the boss’s nose.
The spheres move about the platform and have insane damage output – best to avoid them at all costs! Aside from dodging the purple sphere, you need to continuously attack the tentacle erupting from the porthole.
You need to attack the tentacle constantly until the boss ceases all its attacks and puts out a large tentacle outta its head. As soon as you see it, swim to the top and hit the tentacle until it explodes, commencing the second round.
Similar to The Mighty Octostomp boss battle, the platform is cleared off all ink at the initiation of second round. Therefore, you’ll once again have to create an ink path to reach the boss.
During the second round, the purple spheres are hard to evade, but you shouldn’t face any difficulty as long as you’re standing near the boss. Avoid the purple ink and destroy two tentacles on the boss’s front and back to expose a large tentacle which you’ll need to destroy to start the final round.
For the final round, you’ll have to destroy 4 tentacles around the boss. The best way to do so is: run around the boss while inking the platform and targeting the tentacles at the same time.
Oh, and don’t forget to avoid the purple spheres. It may sound a lot but is easy to execute once you’ve gotten hang of things. With all 4 tentacles outta your way, destroy the single large one to defeat the boss.