Help -4 kings- DS1

4th day of my 1st playthrough .
Started with Thief.
LVL42, 30endurance, 20 str, 20 dex
i have:
bandit knife +5
zweihander +5
Jagger ghost blade +2
knight shield +5
Elite knight armour +3

I'm struggling a lot with the 4 king boss.
would it be a good idea to upgrade the "crest shield" to +5 and then take it to the guy who offers to make it a magic shield?
description of the item says its specially good against magic so i'd be doubling down on it to have a hyper specific magic shield.
i don't mind grinding for it if thats what i need.
i've killed Sif and The gargoyles already.
haventkilled the hydra nor the undead dragon

Update: killed the hungry dragon. got the large ember. now struggling to get throu the blightown so i can leave the sewers
update2: 7h later... how time flies.... i entered the catacombs and killed pinwheel