Before you battle Saddler I recommend you buy these weapons.
You will need to buy: an Infinite rocket launcher, and a broken butterfly(fully upgraded).
When you fight Saddler, use the Infinite launcher to shoot Saddler. When it say climb don't press X or A go in front of Saddler and shoot him with the fully upgraded broken butterfly (in his mouth thingy). Repeat this about like5 times and you'll see Ada give you a special launcher. when u get it, shoot saddler with it and saddler would be burnt if u shot him. if you didn't shoot him, then you'll have to follow this step again.

I hope you find this hint helpful!
Good luck on beating the game!