Sanctuary Guardian Guide

You will encounter the Sanctuary Guardian shortly after entering the new DLC area and traversing the first white mist. Fortunately there is a bonfire just before going into the arena, and there are usually a lot of summons signs on the floor if you are in human form. 

Before you challenge the Guardian I would recommend having the Thunder Stoneplate Ring which can be purchased from the crestfallen merchant in Sen's Fortress for 15,000 souls. I would also advise players to have a good shield with them, preferably one which has 100% physical damage reduction and a high lightning damage reduction %. I used the Silver Knight Shield which I looted from a silver knight in Anor Londo. This shield has 65% lightning reduction which is helpful when the Guardian does ranged attacks. 

First of all don't expect to beat this boss first time, unless you are with a party of good online players. It's important to try and learn the Guardian's move set, and this can be done quite easily thanks to the bonfire just before the white mist. He uses a combination of ranged lightning attacks and close range slashes, however his most powerful attack is the large lightning ball he will shoot at you. Try to get up close and circle around him, he should then do a string of slash attacks. I found he did 2 slashes quite a lot and would then jump away. It's when he jumps away that you have a small window to use estus, or recover stamina. 

It's quite obvious when he is about the use his lightning ranged attack as you will see sparks around his face, if you time it correctly you can roll sideways to dodge the large lightning ball, however there is no way to tell whether he will fire one large ball or several smaller balls. If you're unsure or don't want to risk it, just block; however you will still take some lightning damage through your shield. 

Occasionally the Guardian will also inhale briefly and do a very powerful breath attack. He usually does this up close, and when you see him inhaling, either immediately raise your shield or roll to the side of him. Each time he did this attack it drained my entire stamina bar, but if you hold the left trigger/L1 even whilst your on the floor, you should raise your shield before he does his second attack. Just after this second attack you have an opportunity to get a couple of hits in.

Successful Method:
The method I used to defeat the Guardian was staying up close, as close as I could get. I was constantly circling him, and each time he did 2x/3x slash attacks and stopped I would land 2 hits and then back away to regain stamina before charging in again. If i took hits from his lightning ranged attack I would get up close again, wait for him to jump away after his slash attacks; and then use an estus flask. Eventually I had the confidence to roll sideways to dodge his main lightning attack, but if you're low on health I would NOT recommend this. 

A final note: I was on NG+ as a Lvl 65 pure warrior with high strength/endurance. I placed my summon sign before the door and was summoned into another player's game in his first playthrough. The boss was significantly easier, so if you're beyond NG+ be very careful! The Guardian's physical attacks and lightning attacks deal HIGH damage, so block a lot and don't become complacent. 

Good luck!