This level is tricky for sure, arguably the hardest out of all OE missions. The biggest problem isn't the waves themselves, it's your ink gauge. If you end up on low ink too many times, you will have to wait to refill giving the octos plenty of time to approach your objective. You HAVE to stop the octolings asap before they'll start jackhammering you with specials, Ink Armor in particular is really bad because it'll mean about 6 Octos will get armored which is bad news when you have an objective to protect.
I personally went with the Splat Roller, and then get regular armor and ink armor as soon as i enter. Ink Armor in particular is useful because it'll instantly refill your ink gauge and give you an extra shield on top of it, giving you more staying power to keep the pressure going. The armors will let you tank extra hits too, just jump at them and flick, or if they're really close just roll over them. It doesn't cost too much ink and will reliably take one out with little effort, or if you're lucky multiple ones at the same time.
You can also snipe off the octos waiting at the edge of the map with vertical flicks, or just roll over them as soon as they land.
Don't be passive, don't wait for them to come to you. Immediately seek and destroy them before they'll have a chance to start spamming specials. And furthermore, keep an eye on your ink gauge and only use bombs whenever necessary. Don't bother healing the objective with your ink, it's too slow to have any meaningful effect. Memorize where they'll spawn and act accordingly.
I just used the splat dualies
nice avatar