This massive boss is very slow but can deal quite a bit of damage if he manages to land a hit on you.
Considering he's so slow, you can read his movements very easily. Go in with your strongest weapon and feel free to two hand it. Stay near his feet and he will slowly lift up one of his legs to stomp on you. This can be easily avoided by rolling out of the way and while he's recuperating, you can land one or two hits on the opposite leg.
His other methods of attack include swinging his arms out in front of him and also to his sides, falling flat on the ground (which can do massive damage if he manages to hit you), and ripping off his own arm to swing at you at the halfway point. These are all easily avoidable if you position yourself directly underneath him and roll when necessary.
Killing this boss nets you 'Soul of the Last Giant' and 'Soldier Key' as well as 10,000 souls