Boss 1: Fought aboard an airship, meaning you can fall to your death. Has both draw-in and knockback attacks. Most important thing to watch for: "Void Blizzard IV." When you see this, there will also be a mine marker with the rings radiating out from it. A block of ice will spawn where the marker was, immediately followed by a knockback from the opposite side. You must be positioned for the ice to catch you or you're going over the edge.

Boss 2: Summons atomos adds in several patterns. Move away. After Atomos despawn, there will be explosions that are very powerful and only give a 1/2 second ground warning. At several points, a set of pools, 1/2 blue (water) and 1/2 red (fire) will spawn, and everyone will get a warning message that they are weak to fire or water. Whichever one you are weak to, you need to run through the pools of that color to change them to the opposite color. (No need to stand in them, running through is enough.) Note healers should top everyone off, since even the opposite attack you get is quite strong.

Boss 3: Very. Very. Annoying. Summons hand adds that tether to a target and need to die before they get to them, dark orbs that chase a targeted player, falling meteor particle beams people need to stand in, A massive wave of multiple adds, and several undodgable aoe's Info needed: There's an attack that turns the floors to shadows and puts a shadow debuff on everyone that slows movement and does dot. Sometimes it wore off before killing me, other times not. Not sure what you're supposed to do, stacking didn't work though.

Boss 4: Phase One: Pushover, just kill the deathgate adds fast.
Transition Phase: Kill the Lifegate repeatedly, kill deathgate adds fast.
Phase Two: Basically, this phase is everything this boss did in phase 1, plus almost everything boss three did during that fight. Add need to go down fast, of course, and there is a special gate that spawns at the center that I think the tanks had to soak. Watch for shadow orbs to be soaked as well, and a massive orb that requires you to run to the outer edge to survive.

My estimation: Easier than Weeping City, first and third bosses are the hardest.