After reading through the thread, I'd like to add how mobs under the dark element do not fall for sleep spells. All undead do not sleep (Ghouls and Ghosts and things of that nature) and as mentioned by Bluefrost, the Imp does not fall for sleep. Never cast sleep on them cause it's no good, keep them well silenced, paralyzed, slow, Dia.
With a good amount of MP ready, RDM can tank for 1 round.
In the event of an Imp managing to take down the tank, the best coarse of action is to let the RDM take all the hate. Chainspell cure to those who need it, cast Aquaveil and pop 1 or 2 hard nukes on the Imp. The Imp will come for you, keep up all enfeebling and continue to maintain stoneskin with Phalanx and put up a blink when you have nothing better to do, but always maintain that refresh cycle, and forget about the haste cycle in this critical moment, only keep Haste on yourself.
After a few minutes, When you need to convert, announce that you will convert soon ahead of time if there is anyone in the PT that can cast cure, in my experiences the second i convert after announcing it ahead of time, i see my HP sky rocket when i do convert. Even though you can convert and heal by yourself, it's still better to have that extra cure from a PT member in this case. Cause you'll need to keep up the enfeebling and enhancing. Let the Imp hit some of the others if you need a few seconds, but take back the hate.
Chances are you'll die, but the RDM is better at tanking than any other Non tank class job, and RDM can buy more time for the surviving PT members to give it their all.