To start this battle, get in a few swings at the Gaian's foot until it goes down to about a quarter health.
Run over to the far left and take out the first wave of reinforcements, then stay under the overhang and don't engage the Gaian. Remain there until the reinforcements timer hits 0 again. If you need to, just pull out your shield to avoid damage from the wave attacks.
Although it will be boring, just stand under the overhang and wait for the next wave of reinforcements to arrive -- they won't damage as you they come out of the overhang. Take them all out, then finish damaging the foot to get Ione to jump down and fight.
Parry Ione repeatedly, then dodge roll forward when necessary to take out the wave of reinforcements. Always wait to take out all the reinforcements before going back to hit the giant again. If you let the reinforcements start to the swarm the area, this battle becomes impossible, so its a waiting game without a quick victory.
If you don't like that strategy, there's a sort of "cheat" to employ here. You can actually jump onto the blade, wait for him to lift it up, then jump over to the arm and kill Ione up in the air! After destroying her, just jump down and hit the foot to end the battle.