General Tank Guide (Does not include many Paladin/Warrior specific information)
I believe a lot of "tanks" don't understand the basic concepts of FFXIV tanking so here is some simple tips that may help struggling or new tanks:)
Knowledge, prior planning, and communication is key to successful tanking.
There are guides/videos all over google!
There are class guides, dungeon guides, primal guides, and raid boss guides. Use them it takes 5-10 minutes.
Be proactive in teamspeak, mumble, and ventrilo especially with your off tank and healers. Communication is key to plan tank swaps, cool down usage coordination between you and the healers, and coordinating debuffs on the enemy. Do not be afraid to ask questions either if you are unsure of something.
Tanks are very dependent on gear. Even while doing dungeons while still leveling your healer will love you for actually having a bit of tank gear. A level appropriate weapon helps greatly with threat issues for example.
Your primary job as a tank is to:
Maintain enmity.
Know your abilities and what enmity they generate.
http://valk.dancing-mad.com/tables/enmity-tables/
Do not under estimate keeping your damage buffs up, more damage is more enmity.
On packs of enemies your first priority is aoe enmity. Ensure you position enemies so that you can quickly use and maintain aoe enmity on all of them. An easy way to do this is too hit an enemy caster with a ranged attack then drag enemy melee to that caster. You can also line of sight enemies into a safe area if needed. If you break line of sight from an enemy they will move until they can "see" you again. This also applies if you move out of their range.
On bosses your first priority is your max enmity combo. See enmity chart linked above. (Fast, Savage, Halone or Heavy, Skull, Butcher's). When you need to provoke off another tank or player use Provoke then if melee Rage of Halone / Butcher's Block if ranged Tomahawk / Shield Lob. Then immediately do max enmity combo.
Watch the enemy list, it is a great tool to show current enmity on all mobs. You can even switch targets fast and accurately using it.
Flashing Red: You currently have top enmity on the enemy. You always want to be flashing red, the other colors/symbols orange, yellow, and green are for healers and damage dealers to gauge and control their enmity.
Also the bars on the right side of the party frame will show you how much enmity that player has on the enemy your currently targeting. A full bar for you and an empty bar for healers and damage dealers is the goal for a tank.
Survive.
Choose your gear wisely, generally vitality>all
Do not underestimate your damage reduction buffs, use them at all times.
Use your cool downs pre-emptively, fast, and wisely. If you know that you will or are taking huge damage fast use some cool downs and give the healer time to heal you. Know how and when to use your cool downs to handle certain boss hard hitting specific burst damage as well. Coordinate yours with any the healers may be using in conjunction.
Dodge or stun aoes. Watch for aoe markers and move or stun. This greatly helps healers conserve mp and reduce their threat. If you move when the marker goes away move back immediately, the animation does no damage if you were not standing in the marked area. Melee will greatly appreciate this as the enemies facing will not change. This will increase their dps and reduce the chance they get cleaved and die.
Watch for cast bars, then stun accordingly. (Note: Some enemies require you to know when, and how to anticipate and save and use your gcd/stun for certain casts).
Position enemies/boss correctly.
Know prior to pulling where to position enemies/boss, and if re-positioning will be needed.
Always face enemies away from party. To face enemies away run through them then quickly turn around.
Always face your enemies for max damage reduction, block, parry, and dodge.
Be Situationally Aware:
The party is following you around the dungeon/raid usually. Do not lead them into a wipe!
It is not a bad idea to glance at your healers gear at the beginning of a party. You may also want to ask them if they are confident in their healing ability for the current dungeon/raid. Maybe you can chain pull 3 groups but maybe not... Never rely purely on your healer for your survival, it is your job to assist in your survival too.
Look and know where the nearest enemies are and where your party members are. If they accidentally get some extra enemies do not lose control just establish enmity and use a cool down as necessary.
Be aware of any roaming enemies, they can very quickly chain pull all nearby enemy packs. It is better for you to pull them early and control them or usually they run straight to the healer.
Be extremely mindful of enemies cast bar and animations they will greatly assist you in dodging/stunning avoidable damage. They also inform you when you need to use that cool down/action to reduce the incoming burst damage that will/could kill you.
You also should be watching the other tanks debuff stacks for most fights that require a tank swap. Setting the other tank as your focus target is the usual way to monitor stacks.
