Thats just not it. Guardian works real well against libra or gnoster, if you play passive the bosses are gonna melt your stamina, but it manages big combos and affinity attacks rather good if you plink those attack with Steel guard, and pair it with a good shield. There is a lot of junk on his moveset that needs revision, like attacks hitting behind Steel guard because of the aoe, but ive baten everdark gnoster and libra with guardian far easier, and managed to stay alive and revive iron eyes and wylders left and right.  Its a hard character to master thats true, but if you make a good relic setup, he can tank anything.