Hardest part is the first phase when they stick.
Magic attacks can still penetrate his shield, but the damage will be lowered.
This is an area-of-effect shockwave attack, and it's also the beginning of a four-hit combo, meaning it's deadly if you get caught out by this first strike.
To prepare to fight every boss, make the most of all the classes, and the most efficient way to use your magic and travel the maps, check out this Elden Ring gameplay and guide video to give yourself the help you need to succeed.
In phase 2, dodge through his flying charge attack to as to not get hit and beware of his tail swipe, which has a high chance to be triggered when he is attacked from behind.