Since I have been solo'ing LoDD for the past 3 months and farming it for the past 12 months.
I can tell you that you need ~35k Pdef & ~20k - 35k PA to successfully do it in a group.
You need a bare minimum of I would say ~20k - 40k HP to ensure he doesn't kill you.
In order to do this boss fight.
Once you enter his platform, there will be a mob once you kill will start the boss fight.
At the door of the boss 3 mobs will spawn one of them is a Sealer who drops an unsealer (that allows you to unlock the boss), you need 4 unsealers to open the chains on the boss.
10 sealers will spawn over a period of ~120 seconds.
Once you unlock the boss. Guards & Rune Repair will appear as well. Make sure you have one person (or two) to kill the Rune Repair men otherwise a General will spawn and rofl-stomp everyone (he drops a card btw.).
During this boss fight, Gestro will do a couple spells you need to be aware of. He will cast some sort of fire breathe that leaves Fire on the ground that damages you for a small % of your HP.
Another thing you need to be aware of is when he casts a spell called Purgatory which can instantly kill the entire party.
In order to avoid purgatory, you need to go next to those green Runes and click on it as he casts Purgatory and it will protect up to 6 people.
And if you don't have enough DPS, just continue to walk around the platform to as he will cast Purgatory many times and the Green Runes will respawn after like 45 - 60 seconds. (There are 4.)
This boss fight is fairly easy if you know the strat.
Any more questions, feel free to ask.
