Woot woot
My observations thusfar for Ghost Fort/Merasmus:
1) Merasmus is far harder to kill than previous mobs. He doesn't go up in levels with each appearance (a la Monoculous), nor cause stuns (a la HHH), but his basic level is enough that he's quite tough to kill. Also, he sprays high explosives in all directions.
2) Both teams MUST work together constantly in order to deal with him. Everybody on both teams needs to be ready, with full health and ammo (and hopefully some Kritz or Buff Banners charged) the moment he spawns. Even with this, he is still liable to escape, because he has the ability to teleport anywhere on the map, and can turn himself in to props (like prophunt) multiple times per spawn. He usually becomes around 5 different props each time he hides. Find them and destroy them to bring him back.
3) If you want to be successful at beating the boss, there cannot be fighting between boss spawns. Anything that results in people being injured, low on ammo, or not present at the point the moment he pops in means you aren't going to beat him. If you are deliberately attacking other people, you are causing the server to fail.
4) Sometimes you will be "gifted" with a giant bomb for a head while fighting Merasmus. If this happens, charge in to him. The blast will make you invulnerable, and will stun him, which causes him to take automatic crits from all sources. If you don't reach him in time however, you will explode and die.
4) The most effective classes seem to be Huntsman Sniper, Brass Beast Heavy, and Direct Hit Soldier. Some folks have been using Engy with wrangled L3 SG, but that's very dangerous because if the Engy dies, there's a level 3 SG that wipes out half the server, and that's an instant failure for everyone. Pyro, sadly, is not effective (and causes lots of damage to allies who are on the boss). Demo is exceedingly dangerous to allies as well, as are non-DH soldiers, because of the blast radius of their weapons. Medics can do a bit of damage with their needles after deploying a kritz or something, but survivability is low. Scout scattergun, and all shotguns, do great damage, but the super slow reload means their DPS is too low for long term effectiveness. They're great during the prophunt portion, however. There is no reason to play Spy.
5) Gifts spawn at random intervals, and are personalized - only you can see your gifts. They tend to spawn in the first courtyard area to each side of center - usually in the lower portion, though I've seen one or two in the upper. I don't believe I've yet seen one in the back courtyards or the catwalks.
6) When Merasmus is defeated, he'll fall to the ground and a portal will open. Jump in to the portal and you'll be on loot island, almost exactly like last year. You'll get Merasmus' skull hat as a prize for exiting back to the overworld via the exit portal. Be careful about standing too close to him as he collapses. Some folks have had trouble getting stuck inside his model, which then keeps them from going through the portal.
7) Other than the gifts, there are random drops of random "voodoo cursed" stuff. This works like the burned Pyromania items. Craft any 7 voodoo cursed items together to get a "Pile of Curses." Place this item in your action slot, and then use it. It will give you a random halloween event class headpiece (the Cursed <Class> Soul items). Other halloween items drop at random or come from gifts. Most gifts sadly seem to give the temporary potions and spells, though they can also give the other permanent halloween event items.