Shiftee touched on most everything of importance in this trial.
It is the #1 trial you absolutely have to win 100% of the time if you want to beat the boss at the end.
He mentioned lots of great tips for keeping paladins alive / tanking, but I figured I'd weigh in a bit for group setups.
You *should* (in my opinion) be sending 18 players into that trial. As it is the most important it should be the most stacked. Personally my guild sends:
Tank Group = 2 Clerics, 4-2 Paladins, 2-0 Warriors
DPS Groups x 2 = 1 Shaman, 1 Bard, 4 Monks, Zerkers, Rangers, Rogues, Beastlords
That trial has the following buffs by class:
Basically you want atleast 2 Bards in their because they get a huge DPS mod (they get more in Black, but that trials penalty for not winning is less severe). If you have atleast 2 bards in your roster they should be there. Having 1 bard who is smart enough to use GTT and cure, can handle all the emotes in there using 1 GTT line to tell him who got the debuff and needs cured.
You want to toss all your paladins available into the trial (IMO I know some guilds don't do this though). We have used 2 to tank the bosses and 2 to tank the adds -- perma stunning the adds makes tanking them easy + no blinds. So if you have 4 paladins to toss at it, try it out.
Your clerics do have a huge aggro modifier in there so silent casting to start may help out (unsure on stacking issue here). Giving paladins Shining Armor also will help out as those heals are modded by the heal bonus + stuns (on adds again is win) are more aggro even for the boss.
Necromancers are solid in there however it is also huge aggro so they need to FD twice as often. Giving RB to the tank group with paladins definitely does help but could cause aggro issues potentially if one paladin happens to get lucky and fire it 20 times while the other never fires it. We don't use necros in here that often but they are a solid choice.
Most melee DPS have really bad penalties in here with high aggro or low hit DMG modifiers, Berserker prolly being the best DPS class in there however again huge aggro + defensive penalty. Monks have a defensive bonus but aggro penalty and huge DPS penalty, so they can function as temporary tanks under disc but they aren't too reliable at it. Rangers... I'm not sure how the hit dmg mod works exactly but I *think*? their bowing is exempt + they don't have any aggro mod. Rogues have low aggro but lower damage in light.
Of the other classes not mentioned, reasons I don't use them in there:
SKs are needed in Shadow trial, no reason to put them here.
Druids are needed in Nature trial, they can replace a cleric if need be however.
Wizard/Mage are needed in Arcane trial, plus not many guilds are stacked on these to the point of having excess over 4-6 of them (which is roughly what the arcane trial will need to win, can be done with 3 though)
Enchanter are needed in Arcane trial since its event this expansion you really want more than 1 enchanter. Arguably Fumerak also but meh.