I'm really weird and have a strange idea in my head that if I'm leveling a Guardian for DPS purposes, I may as well roll a Warrior who can do it better. Of course, I'm working under the assumption that GS Guardians are focused on DPSing which is probably not always true. Greatsword Guardian for me is "jump in, pull everyone in, spin-to-win and everything dies."
I like doing dungeons because I'm working with a small group and they tend to be less chaotic and mindless than a zerg. Don't really know what else is considered end-game PvE in this game. My Guardian is in full exotics already so I'm just running dungeons for the experience of working with 4 other people and seeing if we can succeed.
http://www.guildhead.com/skill-calc#MzcmMV9MoLvwooLvxmG9MGxoaMkcoV
Check this build out for you. It's kind of a mix between heals and support, really. (only because heals become attached to a lot of the stuff you do).
Main weapons are Scepter/Shield, offweapon is Staff.
Use scepter to long-range behind your allies, always position the majority of them ahead of you while you tag the enemies. Stand in the back as close as possible to them. Use Scepter 3 to lock down enemies to a spot to make this easier. Scepter 2 and Scepter 1 should be used to deal damage to enemies while you're running around - they both have a great combo and Smite has the low CD. Shield 4 whenever it's up to put up Protection on allies - Use Shield 5 in the middle of your friends (double tap it - don't just use it to block projectiles, get the heal out of it) for a quick burst of heal and damage absorption.
Staff is the best alternate for AOE damage and the Might stack/Heal on Staff 4. Staff 3 is for getting people to move around faster - drop it either as damage or to get melee to move around and position themselves easier. Staff 2 is for a quick heal (move forward, double tap it so it detonates right on the targeted mob), And Staff 5 is to keep enemies from reaching you in the back. Remember that Scepter 1 is a projectile - meaning that Staff 5 will combo with it and cure conditions as it travels. Swap back to Scepter when Staff 5 is used.
Healing Breeze is the better heal here - since you're behind your allies most of the time, you will be healing them in front of you (or you should be running up to them breathing on them like Torch 5).
Utilities are Purging Flames (targetable with trait now), which cures conditions, so you can use it offensively or defensively, depending on the situation. Hold the Line, for Protection and Regeneration; And the Bow of Truth, which cures conditions while it's up, or you can hit the button again for a "healing rain" if need be. It does it at the position it's at, so move into your allies and pop it. Best part, is that it doesn't kill the weapon, meaning it will go back to curing conditions after its animation.
Elite can be either Renewed Focus or Tome of Courage, it's up to you which one you'd like better. I'd personally use Renewed Focus so I can pop Courage again and get double the Protection duration. The key is to stack Protection hard on everyone all the time, making everything do less damage overall. Heals will flow based on your playstyle, so everyone should have a much easier time with you. Never stick in Staff for too long unless you're AOEing. Scepter/Shield should be up all the time, and you won't have to worry about cooldowns since Scepter 1 and 2 are fast and active.
The traits speak for themselves - they allow you to keep the shouts/consecrate/spirit weapons up for longer, better, and more use. I took them instead of other traits because even if you use just one, it makes it amazing instead of using all shouts, all consecrates, etc. Utility is the name of the game.
Damn, this build is so good, I might even try it out haha. It is the most "stand back and protect" build I've ever made.