Guardian of Nature is a great off-tank, utilising incredible AoE damage and self healing, while buffing the group's damage with Embolden.

While leveling, attribute points should be spent in approximately a 1 Strength : 1 Agility ratio as its very important to get max attack speed early on a Guardian of Nature to capitalize on Tremor. 

Once you have 400 Agility / Attack Speed, you can now focus on a more specific build:
Tank
3498 Strength

The current meta on a Guardian of Nature is the full-strength build. This build gets enough Agility from items alone and does not need any attribute points be spent on Agility at max level.

At max level, the core setup is                            .

1.       Immortal Armor of Hepheastus - This Relic offers survivability, mana regeneration and spell power. Easily the best choice to pull you through the difficult fights, and the only real choice for a GoN tank which depends on the static mana regeneration from this item.

Hybrid

49981 HP

With the Hybrid build you still have enough HP to survive crits, and enough regeneration from Vivid Regeneration to continue tanking. Further, you have more mana, and mana regeneration from the increase in Intelligence, so you are no-longer restricted in your Relic choice.
This build brings a bit more flexibility to the meta build, and improves the AoE damage from your active abilities, but reduces your over-all single target damage.

At max level, the core setup is                            /       /     , combined with your choice of Olympian Relic;

1.      Immortal Armor of Hepheastus - This Relic offers survivability, mana regeneration and spell power. Easily the best choice to pull you through the difficult fights.

2.      Searing Flame of Prometheus - Searing Flame provides the best damage output but sacrifices survivability

3.      Thundering Hammer of Zeus - Hammer of Zeus is another option for a more support-orientated build

4.     Towering Shield of Athena - Incredible damage mitigation with a little bit of extra damage from the Strength, AoE armor buff, and a on-use effect to fully heal and replenish the mana of yourself, and all nearby players. Extremely useful for when the healer is OOM!

Caster

24981 HP or lower

This build should not attempt to tank

This build is strictly for AoE damage and support through the use of the Guardian of Nature's active abilities

At max level, the core setup is                            /      , combined with your choice of Olympian Relic;

1.      Immortal Armor of Hepheastus - This Relic offers survivability, mana regeneration and spell power. Easily the best choice to pull you through the difficult fights.

2.      Searing Flame of Prometheus - Searing Flame provides the best damage output but sacrifices survivability

3.      Thundering Hammer of Zeus - Hammer of Zeus is another option for a more support-orientated build

4.     Towering Shield of Athena - Incredible damage mitigation with a little bit of extra damage from the Strength, AoE armor buff, and a on-use effect to fully heal and replenish the mana of yourself, and all nearby players. Extremely useful for when the healer is OOM!

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For your familiar skills, you want to invest in the 5% Health buff from Aptitude for the Tank and Hybrid builds, and the 5% Spell Power buff from Aptitude for the Caster build, along with your preference of 75 Strength/Agility/Intelligence from Gift of Endurance/Dexterity/Wisdom, to achieve the Agility and Strength breakpoints above.

>>Blanket Disclaimer: You, and every other player will need items from all crafting professions so there is no 'best' profession for any given class. If you are selfishly looking for personal enhancements, Alchemy is the only profession whereby you can't bring items you crafted through to another game, so this might be the best choice on a personal level, but increases dependency on others for crafting<<