This is actually quite simple, I just hope you have uber reflexes... (Reaction commands)
Always stay about mid range distance away from him, don't stay too close. Aim drill his HEAD with the strongest bullets you got. Eventually his mouth'll hang open, do NOT knife it, SHOOT IT with the most powerful gun you have. Make it quick though, he's not a chicken. (His mouth won't hang open forever.)
Keeping distance can be hard with this bastard seeing as the moment you turn around and turn again to face him he's right up on you. He moves freakishly fast. If you can't keep your distance, there's another easier, but prepatory method, of which you've most-likely missed out on already. If your anything like me, you're an herb-gathering fool. Make sure you have as many full health regenerative herbs and first aids you can. Ex: Instead of wasting 3 green herbs for Green herbx3 make 3 GreenxRed herbs.
Saddler has a move, which actually helps you, you MUST have an eye for it though, if not, you'll mistake the move and die. Now I haven't played in awhile, but I'm pretty sure he swings his tentcle arm at you at close to closer mid range. If you're caught with a direct hit, rejoice. He'll wrap his arm around your neck, pull you close, and try to strangle you, and... (gasps) what's that I see? He's showing you his eyeball, that's what! Now hopefully you're good at button mashing. If you are, you'll stab him where it hurts. You do gradually lose health though, so get good at button mashing to reduce how much time he's strangling you. It hurts him more than it hurts you. Thus, the herbs and first aids... Repeat the process, eventually, (if he hasn't already died from stabbing him...) he'll recoil and his mouth'll hand open. Repeat the first tactic. Nail it as much as you can with the most powerful gun you got.