As soon as the battle begins get the hell away from Maria. She's dangerous up close and it's hard to shoot her.
Notes on Maria. Honestly Abstract Daddy is harder than she is. Her attacks are very easy to dodge. No need to worry about losing to her at all.
She has 2 main attacks.
A long distance attack in which she sends a horde of black butterflies or moth (I can't tell) at you. It's easy to dodge just runaway as fast as you can until they disappear.
And a close quarter attack which she uses a tentacle to strangle you. If the butterflies/moths caught you then she'll come close to you and strangle you with this attack.
I chose to use the handgun at the beginning of this battle. It was keeping Maria from getting too close to me too quickly. So I was slowing her speed down with the handgun. But of course that wasn't doing much damage to her.
I eventually switched to the rifle. The problem was once her health is at a certain point she'll keep on sending out her insect horde at you and you'll be dodging them more than you would want.
Keep doing the same thing you've been doing for all the boss battles so far. Run to a corner and start open firing at the boss. If she uses the insect horde run around until they disappear and start shooting her again.
Eventually she'll fall to the ground and mutter "James..." over and over again. Aim at her with your gun and fire one last shot.
And congratulations! You've reached the ending! The endings will be different depending on how you acted through out the game. Enjoy your ending! :)