How do you beat Endogeny?

I am playing Undertale at the moment, and I am doing the True Pacifist Route, (Post-Genocide). I am up to the true lab, but I am stuck on Endogeny, the dog amalgamate. Is there some sort of trick that I am missing? I do not know how to dodge the attack where the dog turns into some kind of rocket.

There are two ways to spare Endogeny:

Just like the fight with greater dog, there is an order of ACTs you have to do:

1. Beckoning
2. Petting
3. Playing
4. Petting
5. Petting (again)

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.

The other way is to use one of the following items:
The Hush Puppy, which can be obtained in the MTT Resort at the last door to the right. The person will trade it for a Hot Dog. These can be bought from Sans at Sans' Hot Dog Stand in Hotland.
The Stick, the weapon you begin the game with. When kept in the inventory, you can use it like an item in any of the dog fights to instantly pacify them.
Both of these items will pacify Endogeny and you can spare him.
If you wish to achieve the yellow text for the Amalgamates in the credits, you still have to do the ACTs as described above, but any of the two items will make this easier as Endogeny will not attack anymore when pacified.