Tanking is a frustrating thing at the moment. In newer content, I can keep bosses glued to me absolutely no problem.
To properly single tank the taurus and demon, active hate must be kept on both targets. The way I choose to do this is that my current target gets Flashes (because my flash can be as low as 10 seconds), and the other target gets Provokes. Sometimes, I'll also use the Provoke for the main target.
Even splitting enmity like this, neither target takes their attention off me. I am a well-geared tank. I TP in a lot of enmity gear (secondary to capping defensive stats) and have a fantastic swap for Provoke and all my other abilities. I have a great haste/enmity set for Flash and Reprisal.
I know how to use sentinel, divine emblem, warcry (and rampart if it's not strategically needed). I have a strong cureswap, piled with enmity. In higher content, I can typically save sentinel for when I need it as a damage-reduction cooldown. In lower content, I pop it very early in the fight and right-away if it's mobs that have natural party-hate like Incursion, Vagary. Ambuscade also has party hate, but I just don't need to sentinel as a hate tool in there.
That said, I find myself going to HTBFs sometimes where enmity is a whole lot harder to hold. This is partly because most of this stuff has lower evasion, lower defense meaning the DDs hit harder. This is mainly because of the design of the enmity system.
The lower level the target is, the more enmity is generated by damage, while enmity from job abilities and healing remains the same. That's why DDs pull off tanks in older content so easily. This also makes enmity-cap, which is still a thing, more easily attainable for the DD and once at enmity cap--there's very little that can be done by jobs other than BLM, DRG, and THF.
The thing is, just because they outgear the content doesn't always mean they're able to tank the content.
The theory is that Square Enix considered that if enmity wasn't higher for DDs in older content, like Delve, then DDs wouldn't be able to tank in content where a tank really wasn't needed but that's pretty broad.
There's a lot of content that we outgear where a tank still is needed or is desirable, like many HTBFs.
If that theory is the truth, then how about changing it so that elemental damage and healing did reduced enmity in older content and put melee back on a flat value regardless of how old the content is.
One theory concerning high-tier battlefields is that the harder difficulties don't actually increase the level of the monster, just the stats of the monster. This is effectively similar to how we're still 99, but our gear raises our level. Essentially, our ilvl is calculated in the enmity formula but the monster's "inflated level (set by chosen difficulty)" is not.
It's crazy that damage that would never pull a Reisenjima boss off of me will pull HTBF Ultima off of me (Ultima is the breath one. Omega, I understand, because he has hate resets)
The present design has everyone else enjoying the fruits of their gear progression and tanks working harder and still failing in older content. Since enmity now affects how much hate is lost from taking damage, simply switching to Sulevia's (which has no enmity on it) is not necessarily an option either.
Even with my Burtgang AM3, I'm not going to TP nearly as fast as the blu/dnc/thf/sam/etc. That also means building up tp to 300% which can take a PLD quite a while.
I'll be honest, it's frustrating and embarrassing. People that don't know what's going on think the tank is "fail" and like to spout things like "I thought you were the tank." It takes joy out of the game temporarily to experience this. I'm actually geared enough that I can, if I give it my all, usually hold an easier NM for most or all of it's health in easier content but I get a lot of tells about other PLDs.
I've thought about posting this thread for a while but it's embarrassing to call a spotlight on yourself. It was only once I saw other tanks--that I respect--acknowledging this that I decided to bring it up on the official forums.
TL;DR: Hello, my name is Omnys and I sometimes have enmity problems in older content.