I’m gonna assume you know the basics of raiding, and this is just to help you get a jump start on the Hellfire Citadel raid.

So this boss isn’t really a boss, but waves and waves of adds. You kill the adds, and feed crates that drop from certain dead adds into your cannons, and that’s it. After feeding a certain amount of crates into your cannons, they knock the gates down and you’re done. So on your boss frames, you’ll see the health of your cannons and the gate, and the health of the gate goes down as you give your cannons more crates. The health of your cannons can also go down if you let the waves of adds kill your cannons. If that happens, you wipe.

When you kill these waves of adds, you’ll have to prioritize targets. There’s a mini-boss at the start that you have to get to 50% health, she casts cones of bad that you move out of. She also focuses random members to do splash damage on them, just move out of the range of those who have it or don’t let anyone be in your range if you get it. At 50% health she pulls a Forrest Gump and runs away. Then for the rest of the fight In it’s siege engines > berserkers > engineers = felcasters > dragoons. When the felcasters reach low health they metamorphasize into terrors, and then they must be top priority over everything. The siege engines drop the crates, so that’s why you wanna kill them first. The berserkers put a debuff on your tanks that can become dangerous if you don’t get rid of them fast enough, especially if multiple ones spawn, so that’s why they’re high priority. The engineers repair the siege engines and stun people and drop bombs around, so they’re more annoying than anything else. I suppose they’re dangerous if things are killed slow and then the repairs aren’t interrupted. The felcasters do shrug raid damage, so they can be cleaved, but when they become terrors (happens at low health!) their raid damage is huge, so then they’re top priority. 

After you kill all this stuff and bring the crates to the cannons for awhile, assuming you or your cannons don’t die, the gate will eventually die and you’ll be done.