
This was done using a Flailing Dragon Warrior Build. The Build isn't required, but having a character that can take hits is strongly recommended. A Resurrection skill isn't necessary for this, though it could save you in a bind. The choice is up to you, really.
Now, your party selection:
Olias/Master of Whispers - MM - This is a must have, the meat shields are irreplaceable. A N/Mo with Heal Area is recommended, as it will help when the Siege turtles come in.
Zhed/Sousuke - Searing Flames - Not required, but the AoE damaging helps out a lot. It may be possible to replace this with another MM, but that could cause further issues.
Dunkoro/Tahlkora - Protection - Also a must have. You'll be taking both Monk henchmen as well, so you can't replace this hero with one of them.
Henchmen selection - Devona, Lukas, Redemptor Karl, Jamei - Both Devona and Lukas may not be necessary, but I found that having both to absorb some damage helps in the long run.
Now that you're set up, it's time to start the mission.
Part 1: I'll bring the turtles, you bring the pizza:
This part of the mission should be extremely easy if your character can take hits, but if not you'll need to be cautious.
Once the mission starts flag your henchmen to one gate and your heroes to the other. Run over to the same gate as your heroes and wait for the first group of Luxon Warriors. Your henchmen are more than capable of taking any of the assaulting groups, and with 3 fresh corpses from this first wave your Heroes should be too.
Now, for your job. Run back and talk to all three Juggernauts so they'll follow you. At about this time the first Siege Turtle group should have arrived. What you'll need to do is run around outside with your newly acquired bodyguards and deal with those cannon-carrying reptiles.
This part is fairly self-explanatory, so here's a brief overview:
1) Rush to Western Siege Turtle.
2) Northern Turtle has arrived, take that out.
3) Head south, cutting through the grove, there should already be a Turtle firing away.
4) Once you deal with that Turtle, another group should be heading towards you. Wait for it then deal with it accordingly.
5) Go back to the Western position, then the North for the last group.
You'll be taking on groups of 2-3 Rangers and a Siege Turtle. None of them can deal large amounts of damage, but with constant fire it can stack up. If you take things slower, though, your Juggernauts will be able to keep up with you and take some of the hits. However, with my Warrior the damage I was taking was completely ignorable, without any armor buffs. As a general rule you'll only need to worry about your life if any non-Rangers are attacking you (but don't get careless).
There are 2 points that can cause you problems here. The first is when you head towards the first Turtle group. Atleast one Luxon squad will be aggroed by the Juggernauts as they're moving towards the Western point. You can either take them out, or ignore them and focus on the Turtle first, either way works. Also, a group of Luxon Warriors will probably come to support the Turtle group, and they can actually deal damage, so watch out.
The second point is at the Southern location. Depending on how long it takes you to get there, there may already be 2 Siege Turtles and 4-6 Rangers awaiting you, and the Turtles will be shooting at your henchmen (thankfully not the Tree Singers). Also a group of Luxon Elementalists generally follow the second southern Turtle Group. While it is possible to take this group with your Juggernauts alone to back you up, it could be difficult. The best idea is just to hurry and get here before you have to deal with 2 Turtle groups.
After you're done at the Southern point there should be no more Luxon squads attacking, so the last two Turtles will be easy to take down (you could even get your Heroes/Henches to help, if you wanted).
NOTE: One Siege Turtle alone won't cause your Heroes/Henchmen any problems, and even two isn't bad. If you do this right, though, you shouldn't have to worry about them dying at all during this part.
Once this is done, skip the cinematic so you can have time to prepare for the next part.
Part 2: Things gone horribly wrong:
This part is rather hit and miss. Sometimes it's extremely easy, while others your group will start dropping like flies. Hope for an easy time, but don't panic if your henchmen start to die.
If you've skipped the cinematic there should be plenty of time to set up. Flag your henchmen to the Southern Gate, and try to place them so Devona and Lukas are bodyblocking the doorway. Zhed/Sousuke and Olias/MoW should be flagged just behind your henchmen (speaking of your MM, if you let enough Luxon squads attack your heroes, he should have 4-5 Minions around now).
Now, you get the Northern gate. The Juggernauts should still be following you, so just run over there and try to look imposing. Meanwhile, flag your Monk Hero to stand close to Mhenlo (in other words, you'll now have one Healer/Protector at each gate).
The first Afflicted wave should be easy to take care of, so don't sweat about it too much.
The next one is the killer. The first boss of the mission, the Afflicted Maaka, will be attacking the Southern Gate. As I said earlier, this'll be hit and miss. Sometimes your guys down there will be able to take this attack without a death, and others they'll be tossed around like rag dolls. Sending your Prot Monk down there will probably help things, if you're confidant that you and the Juggernauts can take the Afflicted with only Mhenlo's help.
Otherwise, what you'll need to do is kill as fast as possible. Go straight for Afflicted Warriors, Assassins, Elementalists and Rangers and take out as many as possible as quickly as you can, and keep a very close eye on your henchmen and heroes. Once there are only mesmers and monks left at your gate, run like the wind to the Southern gate. Target the Maaka and try to kill it before it kills any more of your party, then help to mop up the rest. After this you can return to your gate to deal with the last few ones there. (if you have any Clovers or Candys now would be a good time to use them).
NOTE: If you've made it to this part without any Tree Singers dying, breath a massive sigh of relief because it's smooth sailing from now until the end.
The next few groups will be normal, plain Afflicted, so just deal with them as they come. Unless something very strange happens, like 6k ping lagspikes, there shouldn't be anyone else dying.
Once you get a message saying there is 1 Afflicted group left, get ready. The Northern gate is about to assaulted by 2 bosses. Luckily, if you could tank the Turtles and Rangers easily, you'll be able to survive this too. If you watch your mini-map, you'll notice that only 2 or 3 Afflicted attack the Southern gate. Keep the bosses busy for a bit, don't worry about killing them right now. Once you've seen that the Southern gate is clear, cancel your flags.
With a full party of 8, 3 Juggernauts, Luxon archers and Mhenlo backing you up, proceed to mindlessly massacre the Afflicted.
Congratulations, you've just Mastered the Eternal Grove.