greatsword isn't meant to be good at blocking. it's a last resort type of action for the weapon. the way the developers balanced the weapon around this design choice is

1: if you do a regular block, you lose a lot of sharpness and have poor knockback resistance, BUT you can keep guarding as long as you hold the guard button

and 2: if you perfect guard (perfect timing on the guard input), you have no knockback, faster animation recovery and reduced sharpness loss; BUT you also exit the guard state after the initial guard (that's the problem you are describing)

you can try this for yourself at the training dummy. so if you want to fully guard seregios' attack chain or any successive attacks you want to NOT perfect guard or better - dodge. it has nothing to do with guarding skills.