Well, this is just for fun. No need to kill this boss like this. But, in case you waste your Giant Souls there is a way around. Just be patient.
In this video I used:
* Bleed Bandint's Knife +10 (any other bleeding weapon will do., I had that)
* Crest of Blood (ring. Buff to bleed effect)
* Repair powder (weapon's durability in a +15 minutes long fight will suffer, believe me)
I also recommend having the Shadow Gauntlets. The bleed buff stacks with the ring, so it's nice having them. I did not, so you know they aren't strictly needed.
Other than that, you need to stick to this plan: keep close to his left leg, circling around to your right while slashing his knee between his attacks. Dodge as you need, but most of his attacks tend to miss in this "sweet spot".
He will eventually jump back. You can run to him, or back off to wait him. Also, watch his armor laying around. You may get trapped while circling around Vendrick.