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Just like the fight with greater dog, there is an order of ACTs you have to do:
After these you will be able to spare him.
Dodging the rocket attack can be difficult but is doable. When he begins a charge, he will go for your last position so try to strafe away from him diagonally and try to avoid getting stuck in a corner or against a wall. It is not easy but with this you can minimize getting hit to maybe 1 or 2 times. Bringing some chips from the vending machine or any other healing items will help you survive.
I spoilered the other way to not take away from exploring the hidden things that make exploring Undertale so fun.
Our complete Pacifist guide tells you how to get here, but in short, you need to go through the entire game without killing anything to unlock this bonus dungeon.
True Laboratory is an optional area in Undertale, following the boss battle with Asgore and completing the requirements of a Pacifist playthrough.
Most other endings follow the exact same route and only differ in small ways, and our wider Undertale walkthrough delves into other areas of the game.
The elevator doesnt have power, so leave through the door to the left and follow the corridor. As you approach the switch in the room at the end more creatures will attack, but Alphys will call them off. Restore the power, then return to the elevator and youll be returned to New Home.
Character information: Endogeny is a large canine with too pointed ears on its head. Instead of a face that resembles a canine, they have a dark single orifice. Endogeny has a large wide body with 6 legs, with 5 smaller silhouettes between them. When Endogeny is spared the silhouettes appear to have faces.
How do you spare the Reaper Bird?
How can I spare my memory head?