Trust me,
I made it, it's possible to pull Hydross with imp without getting the add.
Just send your imp to attack, and when it begin to cast fireball, click on "stop" and "stay" buttons. Then turn back and get as far as you can without depop your imp. Then push "fireball" button, when Hydross is hit...run...and just click on your imp and kill it. Then Hydross will rush you, the adds will pop 2 secondes after at his spawn point without aggro you.
This method work about 5/10 try.
Already did it 3 times
Solo it...enjoy your drops.
had a dps war smith pattern damn!