When taking on Saddler start off with the TMP- you only need to hit one of the eyes to make him go down for a bit. Then, once the large eye is vopen, use a more powerful weapon such as the broken butterfly. After a few sessions like this you will get a cutscene with Ada dropping off a rocket launcher on a platform. Once you are in control again get hold of it and blast Saddler with the launcher.
Be careful with your aim- the launcher is a single shot weapon. If you miss you will have to quit, re-load your save and start again with the battle 9not what you want to do, I'm sure).