If it's happening on trash mobs, Quelling Strikes has a 2 minute cooldown and Elusive Jump a 3 minute cooldown. If they're doing enough DPS to rip it off you (assuming that you're not doing just really low enmity), I'd assume the fights aren't lasting that long, so even if they use it on one fight, it won't be back for the next.

If it's only on bosses, unless they're significantly overgearing you, there shouldn't really be a reason that you don't have time to gain enough enmity before they get stuck in unless they're literally hitting it before you.

But when you say you're wearing "tank gear", do you mean you're wearing all Vit accessories? That's the only "tank gear" you really get to choose on because your tank won't be able to wear DRG/MNK/NIN/whatever gear (past a certain point, I'm assuming you're not talking about pre-50 here because you mention Ultros).

If you're having trouble with enmity, Vit accessories won't do anything to help, although it'd be fabulous if they actually reworked enmity generation for tanks to increase with the more vit they have.