During Act I, a quest called Save The Refugees will lead you to saving the Emerald Grove which can be done by preventing a battle or joining it.
You get the choice to side with the grove if Minthara marches to the gate with Goblins, which will end with a bloody battle.
Should you decide to side with the grove, you will need to join the battle or may end up preventing it depending on how you progress with your quests.
How To Save The Emerald Grove?
1) Defeat The Goblin Leaders
There are multiple ways for you to save the grove and one of the easiest ways is to defeat the Goblin Leaders before an attack can be done.
This includes slaying Dror Ragzlin, Priestess Gut, and Minthara within the Shattered Sanctum and this becomes a part of your quest after freeing Halsin.
You can either fight the Goblin Leaders and their minions with the help of Halsin or take a stealthy approach and clear the enemies using stealth.
2) Face Minthara At the Grove
If you have gotten your way into the Goblin Camp and later the Shattered Sanctum without getting into a fight with the Goblins, Minthara can be spoken with, believing you are an ally.
Should Minthara push through with her plans to attack the grove, you will have some time to prepare and may begin the fight after a Long Rest.
This will require you to head back to the grove and start the battle by using the large horn to rally the forces into the Grove.
How To Defend The Grove?
The battle will begin once you blow the horn and this will cause enemies to appear outside of the gate, where you may find hidden oil traps.
You can also gather the oil barrels around the area and place them where your enemies will be as well as any barrels you have found in the Shattered Sanctum.
Placing barrels beforehand and igniting them when there are enemies will give you an advantage during the early phases of the fight.
It is also a good idea to position one or more allies near Arka as she is one of the earlier NPCs that die but can be useful if you save her from a Sword Spider.
Keeping to the high ground will give you an advantage and taking out the sappers from afar is a good way to prevent the gate from being blown up early.
Verdict
The battle at the grove can be an intense one, especially if your characters are not prepared for a fight since the grove often becomes overrun.
It is wise to keep close to Zevlor during the fight as he stands a good chance against enemies while making use of the traps is vital to achieve victory.
Avoiding the conflict the grove can be done by killing the Goblin Leaders in advance, but nothing will change if Minthara has already left for the raid as she will appear after a Long Rest.