Someone might say "I shouldn't need all of this stuff just to do well in a raid". Keep in mind that Blizzard has empowered the modding community to create, and in some cases has even encorporated some mod concepts into the default interface. Also, the raiding game has increased in difficulty and complexity tremendously since the days of Ragnaros or Nightbane, undoubtedly at least in part due to advances in raid mods.
If you have not read up on the bosses abilities in the colliseum, suffice it to say that they are very intricate encounters, where knowing and reacting the status of people in the raid will be extremely important.
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The first list is for everyone in the raid:
1) Install Deadly Boss mods (or Big Wigs). I do version checks periodically in raids, and believe it or not, there are still people in them that don't have a boss mod installed.
2) Disable quest/leveling (Carbonite), map enhancement (Cartographer), and any other graphically taxing mods during raids. This is especially important if you are on an older PC.
3) Use raid assistance mods like Loggerhead and Recount only if they do not bog down your system. For example, before I seriously upgraded my computer, these two mods would make WoW explode in 25-man raids due to the constant disk writes.
4) Install some sort of raid frames where you can see everyone's health (and mana!). It might seem busy at first, especially if you are DPS, but it's crucial to know, for example, if everyone in the raid is at half health before you lifetap. Suggested mods: Grid, Xperl
5) Install a buff/debuff frame that allows you to place your own debuffs in a highly visible and prominent position on the screen. This allows you to react quickly to threats like Light and Gravity Bomb. Suggested mod: Satrina Buff Frames
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The second list is for Healers especially (extra credit for tanks). It requires Grid/Healbot/Xperl raid frames.
1) Install lib_QuickHealth. Also install the QuickHealth interface into your raid frame. This is the best healthbar update library currently available. Quicker health updates let you respond more dynamically to threats, prevent quick deaths, and achieve less overhealing.
2) Add all incoming-damage-altering player buffs as a visible icon in a certain location on your raid frames. Example buffs are: Icebound Fortitude, Shield Wall, Evasion, Pain Suppression, Hand of Sacrifice. There are probably 20 others as well. Being able to see what players have these buffs on them will allow you to make smarter decisions about who (and how much) to heal. I have these as right side icon in Grid.
3) Add all spell short-duration auras and HoT's as a visible icon in a (different) certain location on your raid frames. Example buffs are: Power Word: Shield, Riptide, Renew, Regrowth, Frenzied Regeneration. This will let you know if you don't need to heal someone who is not in danger of death and can be topped off by existing HoT's. This is especially important for Druids, who can keep multiple HoTs on 3 tanks and heal the raid at the same time. I have these as left side icon in Grid.
4) Add all of your self/other combat buffs as a visible icon in another location on your raid frames. Example buffs are: Earth Shield, Water Shield, Inner Fire, Omen of Clarity. This will help you keep their uptime maximized. I have these as center icon in Grid.
5) Add an aggro indicator to your raid frames. it'll keep people from dying to certain mechanics if you pre-emptively heal them. I have this as top left and top right indicator (red) in Grid.
6) Add all incoming-healing-reducing player buffs/debuffs as a visible icon in another location on your raid frames. Example auras are: Mortal Strike, Shadow Crash. This will let you know who needs extra heals relative to their damage taken. I have this as top icon in Grid.
This might again seem like information overload, but you'll get used to it, and it'll end up being an indispensable tool later on. Here is an important conclusion: Doing all of these things will take some time, probably a few hours for the healers. Getting used to all of the extra information could take weeks. These are big steps forward and will reap great rewards for the improving raider. Thanks for reading.

Coming soon: Links to mods, lists of buffs/debuffs, Grid config files.

