Hard to remember the first time but as already mentioned, fast roll is a must. I use the dark wood grain ring for extra i-frames and the wolf ring for poise so I can still block without getting stunlocked. Kindle the bonfire to 20 estus if you haven’t already. Make sure you only have estus and the silver pendant equipped in your items so you can flick between both as required.
I tend to avoid summoning Sif because although he provides an occasional distraction the health boost he gives to Manus makes the fight last a lot longer. If you do summon him, stay close beside him as Manus will be more likely to get his choice of target confused.