If you’re impatient and can’t wait for them to patch this boss later today, here’s how you can beat him now.
Guide to Beat the Mantis Boss
How to Do It
You dodge his attacks and when he goes for a big swing you hit him in the eyes. After about two big hits a handle drops down, you’re meant to pull on it -super- quick to lift the punching bag thing to stagger him. Pulling on it is not essential to killing him, so just load up on auxiliary oxygen tanks and smash his eyes when you’ve made an opening as much as possible. High level harpoon and/or flame sniper rifle are very useful for this.
Try and fight him in the middle ground. If you hover directly overhead he can’t hit you so he backs up. When he does his rush sequence you get pushed back toward the spikes so you want just enough distance from the spikes that you can grab the chain in time for him to deflect and stagger or for you to dodge his attack entirely. You’ll fail at this about as much as you succeed at best so don’t get caught up on the ball and chain mechanic.
When he staggers from the punching bag he has an opening moment but he’s so far back it’s difficult to even get a hit in if you swim at full boost and unload with a long range weapon. Often it’s better to just go over the top of him since the punishment for failure is getting pushed toward the spikes so if you can avoid that and just keep getting those high damage number knocks in you can take him down. The key is just putting in enough points of damage to them eyeballs, and the stagger from the punching bag is kind of useless at the moment for actually giving you more openings and more of a damage hazard, so if you avoid the damage and keep fighting you’re actually in net gain territory.
They’re going the fight a bit easier with a patch later today, but if you can’t wait: Sell a bunch of sharks from your fish farm for cash to get that major spear upgrade if you’re stuck on the same day without it, grab your best sniper rifle, go round the map and find a pair of oxygen tanks, and just wail on them eyes till he falls over. Walk him back with hovering over top of his eyes, dodge around his punch telegraphs, create an opening, get one to two knocks in, avoid the spikes if you can, try to avoid damage from his big attack, pound oxygen as needed, repeat and keep going till he’s dead It’s doable.
Best of luck.