Para que haya una idea de lo que anda mal o bien dejo esto, ademas en el link de abajo se ven las diferencias entre normal y hero, aclaro porque dijeron que spawnea mas de 1 add:

Lord Jaraxxus

Heroic Specifics

General: Nether Portal and Infernal Volcano will now spawn adds until killed. Esto anda
Mistress of Pain: Will now cast Mistress' Kiss on random players. The spell does damage and interrupts the next spell cast within 8 seconds, causing school lockout as well. Vi el debuff en uno de la raid pero no puedo confirmar si corta el cast.
Felflame Infernals: Can no longer be CCed or interrupted. Anda.

This fight will be a little more complicated. Your DPS will need to switch to the newly spawned Portals/Volcanos as soon as they spawn. If you do well, you will only get 1 Mistress or 3-4 Infernals - no different than the Normal version. Of course, both kinds of adds will need to be killed very quickly, as they are even deadlier now.

For casters, make sure you have a spell from school you don't usually use (sorry, paladins, sucks to be you here), dragged somewhere handy. Hit it as soon as you see Mistress' Kiss on you - this way your least used school will be locked, and you will be able to keep on DPSing/healing.

Run away from Infernals that do their AoE. It will hurt more, and this time you can't stop them.

Fuente: Strategist: Trial of the Grand Crusader - Wowhead News.