Fighting the Keeper

Every player should be prepared for the upcoming bossfight with whatever weapons, gobblegums, and shields they need.

Each player should place the Ragnarok DG-4's on one of the blue panels around the pyramid using the left trigger. If you don't already have them, here at the locations.

Killing Pandzer

Activating the death ray, then using the wundersphere to collect it.

Rocket test site, when rocket test goes off, activate switch and go back inside

Once rocket test is finished, activate switch on computer and then the part will spawn inside the teleporter.

Once you have placed the four Ragnarok DG-4's around the pyramid, you will then be transferred into the boss fight

The Keeper will soon spawn along with many skeletons and begin its attacks. Simply survive until the Keeper returns to the middle of the arena, and a ball of white electricity appears below him.

Immediately, a player should place down Ragnarok DG-4's below the Keeper to stun him once he descends.

Then, the symbol on his chest will glow orange, and every player should damage this symbol as much as possible while avoiding death by the skeletons.

If you do not place the Ragnarok DG-4's under the Keeper in time, he will do an attack that instantly downs all players not hidden behind a pillar. So it is crucial you place them down quickly to avoid death.

If enough damage is done, the screen will shake and the next stage will begin.
 

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Make sure you pick up your Ragnarok DG-4's

 

Panzers and more skeletons will begin to spawn, as well as a max ammo in the center. Survive and kill to charge your Ragnaroks. Eventually the Keeper will spawn again, and you should repeat the process of placing the Ragnaroks under him during his attack, and shooting his chest symbol. 

You will repeat this process one more time, as even more Panzers and eventually Hellhounds begin to spawn. With each successive appearance, the symbol on his chest will have more and more health. After the third time successfully damaging his chest, he will be defeated, and you will be returned to the map.

Pick up the Summoning Key from the MPD.

Then place the Summoning Key at the computers at the top of the castle near the clock tower, and watch as rockets destroy the Moon. The cutscene will then begin, and you have completed My Brother's Keeper.