Good luck?
Actually, it had better be an extremely high damage weapon with health, stamina and magicka drain.
If you're not even going to use the bow, then you're in for it.
Harkon's spells are devastating and he heals completely in his bubble, not to mention that he raises the dead, summons gargoyles and is almost impossible to hit melee-wise in Vampire Lord form.
My suggestion, use Serana. When my latest Orc hammer wielder went to face him, I just hit him right away with my mightly blow as he began to talk to his daughter. It took him 2/3 or the way out and I was able to get another in before he could really react.
The only real problem I see with your plan is disrupting his healing bubble and keeping his summons' at bay.
I wonder if Dawnbreaker will work on him?