Guardian is a strong foe. He makes use of some common attacks that can deal physical damage such as Headbutt, Thwack, Roundhouse and Right Arm. All of those attacks can inflict major damage and cause statuses. Guardian is also fond of casting Quake, which will do up to 1500 damage.
His most dangerous attack is Pox, which does a bunch of damage and causes Defense Down, making his next attacks more dangerous.
Casting Protect, Shell, and Haste as soon as the battle starts is recommended. The Mask's Flare can damage him quite well, and Tidal Wave does more than half of that. Constant healing is also necessary.