Cooldowns and when to use them best...

[Sarcatica]

The general knowledge is that dungeons wise, you don't ever need to use CDs for small pulls. However, you would want to pull more than that if you want to get by it faster, usually 2-3 groups of mobs at 1 go. Now, that's when you want to learn how to pop CDs appropriately if you want the best efficiency and pretty much less time killing mobs/bosses in dungeons.

It also depends quite heavily of what kind of DPS composition you are running with. Naturally DPS jobs that can AOE well is going to destroy a large pull a lot faster, unlike a few jobs that are restricted on resources to keep doing AOE (TP issue mostly)

As for tank busters, most of those hit like noodles if you compare those to the hardest raiding dungeons so they aren't as painful. Most of the harder bosses (trials/raid) will have their own patterns on tank busters. Most inexperienced tanks will outright fail because they blow a lot of their cooldowns and not planning them better for the next sets of tank busters.

[Lyth]

You could just enlarge your focus target bar and place it directly under your character on the UI.

As a rule of thumb, you should never hold on to a cooldown for longer than duration of the cooldown timer. For example, if rampart comes off cooldown, it should not remain unused for more than 90 seconds. If the next tankbuster is in 45 seconds, you can hold on to it. If the next tankbuster is in 145 seconds, then you use it, and then hold on to it when it comes off cooldown (55 seconds before the tankbuster).

Because boss encounters are very scripted, with cleaves and tankbusters occurring at set intervals, you can map out your cooldown usage in advance. Once you've identified the constraints on your cooldown rotation (i.e. placement of tankbusters), you can time your unused cooldowns so that they soak up the most damage (i.e. the largest number of cleaves or periods of high auto attack damage. You can also use this principle to decide timings where you can stance dance without taking much damage (i.e. periods of long spell casts without incoming cleaves, or periods where you have defensive cooldowns up without incoming tankbusters).

Outside of bosses, you apply the same principles less formally by using your experience to identify pulls where you tend to take more damage, and compensating for them in advance.