The Leviathan is a rather easy encounter if properly setup. First you will need to clear the zone of all the Fishmen in the zone. The fishmen body drops are skins that you will use to defeat the Leviathan.
First you will need to decide on who will enter the Leviathan. The best candidates to enter are classes with fast cast time, enchanced reuse time, and alot of spell crit. Coercer, Illusionist, Wizard and Warlocks are some of the best but really any class will do that meets the above criteria.
Give one fishskin to each of the guys who will be killing Leviathan. The other fishskins will be given to the high dps melee members of your guild. If you have not been in the zone before pay attention to this part. You will need to farm the bombs before you take leviathan down. The bombs are found on Corpse drops from Gnashers inside the Leviathan. (Be sure when you are ready to farm the bombs that your loot options are set to free for all so the people inside can loot the bombs from the corpses).
Your first time in the zone you should harvest as many bombs as possible to lessen the times you need to harvest them again. The bombs are stackable and can leave the zone.
The safest way to farm and then kill is to farm as many as you can in one pull, wipe, then do the kill pull.
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Time to kill him. When you have enough bombs, give them to the 2-3 people who will be entering. The bombs are clickable from the hotbar as well.
The leviathan has a reflect availability that can be removed periodically by Furies and Wardens. This is important to dispell as often as possible so you can get debuffs on him quickly as possible.
The leviathan has a poison AOE that ticks pretty hard and fast. It is best to have group cure's in every group but if you cant field that many in a raid, you will want to have cure pots. The poison AOE will also hit the people INSIDE the leviathan as well, so be on the ready to cure yourselves inside.
When you engage the Leviathan (do it from his backside where his two eyes are because the people who enter get ported out on this side), his current target must be in melee range of him. Generally your tank can hop into the water and be just fine, otherwise you need to be on the inner rim of his bowl to avoid a massive AOE that damages gear quite badly and wipes your raid. Just have everyone rush in. Your only gonna be debuffing him anyways as his Thick Hide ability reduces all damage to him quite effectively.
The only way to kill him is to enter him and blow him up from the inside. When you engage him, one of your designated bombers will activate the fishskin and turn into a small fishman. Every minute the Leviathan will swallow a fishman, if you dont use the skin, he will AOE the raid down.
Once inside, spam the bombs to your hearts content. A way to maximize the bombs effectiveness is to wear items that increase base spell dmg, the RO miracle that increases heat dmg and causes 100% crits will as well. You will want to setup a cycle for entering as well so you can spell haste whoever is going in to help cast the bombs faster. A Chanter with decent reuse and casting speed should be able to get off close to 10 bombs per entry. The bombs will take off anywhere between 2% and 8% of the leviathan's health.
Your raid might have noticed a large globe in the zone just to the side of the leviathan. A member of your raid must have a Greenmist orb on them at all times for each attempt on Leviathan. At 10% of the mobs health the globe will become active. You will see a event dialog on your screen as well. To get to the globe you need to track along the wall and come at the orb from behind. Be careful because the tentacles flying about can kill you or knock you away.
It might be best to sent your Greenmist orb carrier over to the globe at around 25% health because the health of the mob drops rather quick. You cant attempt to kill the Leviathan until the Greenmist Orb has been activated. If you get to 0% he will heal and AOE the raid. Do not bomb him until you see the text event that the Leviathan is now vulnerable. When you see this you can kill him safely.
Enjoy the loot.