Xantalos has the right idea: we should figure out which tactics will work the best against Gehrman...and at which stage of the fight they should be used.
I'll personally give my seal of approval to Xantalo's idea of using the Old Hunter's Bone, and Zer-Author's idea of breaking Gehrman's Scythe. I feel like these strategies would be best used in the mid-to-late stages of the fight: firstly, so we can blindside Gehrman by changing the nature of our duel...and secondly, so Tricia has the space to write some beautiful descriptions of Gehrman pulling crazy stunts with his scythe.
This is the second-to-last boss fight of the Quest, after all...we need to make the awesome quotient shine high.
Hmmm...here's some Non-Vote thoughts on how this fight might shake out:
––At a key moment, Anderson might be able to pin an explosive chain on Gehrman's Scythe...and blast apart the metal joint that lets his Scythe shaft fold in half. Gehrman will try to retrieve his siderite blade...but since that blade's magnetic, Anderson can seize it with another bayonet chain and hurl it away.
––War sickles and scythes like Gehrman's have an uncanny ability to slip around the guards of people's weapons. When (if) Anderson parries Gehrman, he'll need to either keep far enough back to avoid the Scythe's hooking motion, or get right up in Gehrman's face and jam his arm movements. In medium range, Anderson's gonna lose limbs quickly.
––I highly suspect that using the Laser Club on Gehrman will literally backfire, much in the same way that Alucard's Jackal exploded when he tried to use it on Walter. ("Did you forget who made that for you? That cudgel was made to kill Great Ones, and I'm no Great One.")
––At some point, Gehrman may well shoot Anderson in the face and perform a visceral attack (which we haven't yet seen in this Quest). This could spell Anderson's end...or it could give Anderson the chance to seize Gehrman's arm, grapple him, and finish the first Hunter off with headbutts. Just don't let Gehrman pull out our heart or spine.
––Lastly, when (if) Gehrman's defeat, I can't help but imagine that his death will be more drawn out than in the game. Slumping back into the field of coldblood blossoms, he'll exchange a few meaningful words with Anderson, reveal a few more secrets...and Anderson may even get the chance to say the McManus prayer and shepherd the First Hunter on his way...