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Make it FULL STR
Get all your buffs 7 (except the accuracy one)
Max out your stuns (except the shield one because guardians dual wield dagger 'n sword)
Get roha blessing, crazy standing, OS and Stone skin 7 for pvp purposes
Use frozen as elemental because it will slow people down if they are in your OS+CS combo
Mana drain skill is nice to have 6 as well for pvp purpose
At lvl 50 you get a skill and stat reset stone for free. I'd just say you build your guard how it feels good to you for pve, then at lvl 80-90 you can reset and go more into a pvp build.
Editing to add that Guards arent good for pvp, they rely on their buffs and if a priest, temp or mage comes along and debuffs them they are totally useless. You could get a good guard if you would reach 100% damage drop but then still, 1 debuff and your damage is gone and dffs will still 1 shot you.