The thing about debuffs, though, is that many have to be actively applied. Sure, folks like paladins spam their Judgments at all costs, so the debuff associated with that spell is certainly going to be nearly always present. But what about a rogue's Expose Armor? Does that really have a solid place in their rotation?
If a rogue has to go out of their way in order to Expose Armor (and they do), then you have to sort out how much DPS you lose from the rogue changing their rotation. Is that loss really worth having the debuff present for your raid? Equations like that is the stuff Elitist Jerks is made up of.
Nonetheless, if you can manage to squeeze in all of your debuffs (or even most), your raid is going to have a vast performance improvement. Let's take a look behind the jump, and see who lays which debuffs down on the hapless and helpless boss.
Armor Reduction (20% or 5%)
Your general Armor Reduction comes in two flavors. It can come in the 5% reduction category and it can come in the 20% reduction category. The two categories do stack, even if other effects later boosts the percentage of armor reductions.
The most classic version of the Armor Reduction debuff is Ye Olde Sunder Armor. I remember very well our warrior tank screaming "Wait until 3 stacks before you attack!" Nowadays, though, warriors actually tend to apply the sunder armor effect by using Devastate. Rogues can Expose Armor, but at the cost of 5 combo points. Lastly, a hunter who happens to be cruising around with a worm can have that pet Acid Spit. All told, you're probably best off looking for a warrior, as even the non-tank warriors can keep Sunder up fairly easily.
The minor version of Armor Reduction comes from druids, warlocks, and (once again) hunters. Druids have Faerie Fire, which is really kind of the Swiss Army Knife of the debuff world. Warlocks can use their Curse of Weakness, but I haven't met one yet who likes using that ability. Lastly, wasps can Sting if your raiding hunter happens to be using a wasp pet instead of a wolf like every other hunter. At the end of the day, you'll probably have a druid doing this debuff if you have the option.
Melee Attack Power Reduction
The attack power reduction is somewhat less key a buff nowadays, since it debuffs by a flat amount instead of a percentage. However, everything that makes your tank get hit a little softer is worth it. (Still, it's like the difference between getting hit by a bus and getting hit by a somewhat softer bus.)
Warlocks, druids, and warriors bust out this debuff. Warlocks get to sport their Curse of Weakness again (the same thing that reduces the targets armor), and thus there's a pretty good argument for having your warlock do so. Druids can weaken the hearts of their enemies with a Demoralizing Roar. ("Rawrrrrrr you smell funny rawrrrrr.") Warriors, however, are considering copyright infringement since that rips off Demoralizing Shout.
Melee Hit Chance Reduction
If you want to reduce the boss's chance to clock your tank, you only have two options. Have a druid put an Insect Swarm on the boss, or have a hunter apply Scorpid Sting.
Melee Speed Reduction
Melee speed can be reduced in a couple of ways. Icy Touch is a key ability for a death knight, anyway, so they'll be spamming that periodically. Of course, the same general principle (part of the normal rotation) applies to druids' Infected Wounds. The last two melee speed reductions will only be naturally included by the tank specs of their classes. These are the paladin's Judgements of the Just and warrior's Thunder Clap.
Bleed Damage Taken
In a sense, a good Bleed effect is almost a debuff all on its own. Or, at least, it feels that way as I stand there and bleed to death in the Arena. To add insult to injury, however, druids, warriors, and hunters all have a debuff which will increase the amount of damage you take from bleed effects.
Warriors can pick up Trauma about halfway down the Arms tree, while druids get Mangle in their feral tree. Druids and bears both have a 30% bleed damage increase. If you don't have a druid or bear, though, hunters show up again with Trample. That's a rhino thing, in case you were curious. However, I can not recall ever, ever, ever having seen a rhino in any instance before. Ever. If I happened to see a rhino in an instance, I would likely be thinking less about the debuff and more about "Whoah. A rhino."
Physical Damage Taken (Percent)
Warrior's Blood Frenzy and rogue's Savage Combat will both increase the amount of physical damage taken by 4%.
Spell DebuffsCasting Speed Reduction
Okay. It's important to note that reducing someone's casting speed is not a Slow per se. It's slowing down their casting speed, but I want to make sure I point out it's not a Slow in an of itself. It's confusing because the arcane mage's Slow does reduce casting speed, but it also decreases the target's run speed.
You can also reduce someone's casting speed with a warlock's Curse of Tongues (not their favorite curse, either) or a rogue's Mind-numbing Poison. Hunters are weird, again, by supplying Lava Breath. That one I do see periodically, because the Kurken is just awesome.
Critical Strike Chance
Three abilities debuff a target so that attacks against that target have a 3% increased critical strike chance. Ret paladins roll in with Heart of the Crusader, rogues bust out Master Poisoner, and Shamans lay down the Totem of Wrath.
Spell Critical Strike Chance
Looking to increase your spell-flinger's chance of hitting the boss with a critical strike? You're going to want a mage or a warlock. Mages can work that mojo by hitting the boss with Improved Scorch or Winter's Chill. A warlock will have to lay down Improved Shadow Bolt.
Spell Damage Taken
You can increase the spell damage taken by a boss by hitting it with Earth and Moon (a boomkin specialty), Ebon Plague from unholy death knights, or the warlock's Curse of the Elements.
Spell Hit Chance
Casters have such a high hit cap that it's incredibly helpful to have debuffs that increase the likelihood spells will land. The two ways to supply such a debuff are Improved Faerie Fire from boomkin, and Misery from shadow priests.
Other DebuffsMortal Strike (Healing Reduction)
I know that Mortal Strike (a warrior abiilty in the Arms tree) is not the only ability that reduces the amount of healing received by a target, but the ability is so intrinsically tied to warriors that I can't think of the debuff as having any other name. That being said, warriors have two versions of a healing reduction, with the second stemming from Furious Attacks in the fury tree.
Hunters can provide the debuff with Aimed Shot, and rogues can do it with Wound Poison.
The Paladin Snowflakes
Paladins have two abilities that are kind of unique snowflakes. When they slap a Judgement on the mob, people who hit the mob will get health or mana back. These two abilities are Judgement of Light and Judgement of Wisdom. Since each paladin can only use one at a time, decide which of the two you value more greatly.
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