You don't fail the challenge if you die and get resurrected, you also get full hp and salts back, I just used it on Ops Zeal wave 15 challenge. (however somehow I faiiled challenge for headshots only >_<)
Imho the achievement is manageable with a lot of work/patience/luck (luck is heavily involved in some challenges)
So far up to arena 3 the only challenges I think are very hard are:
5 kills with Devil Kiss: really? how? they don't even follow me! and trapping them in shock trap, sucking them with undertow or airborning them with bucking is only about luck. Tempest gear might be required probably to combo-wombo the explosion
Zeppelin in 1.30: no timer makes it hard enough, those two snipers could screw you if you are not fast, hope for lucky undertows @ start and the youu must kill everyone, break the engine AND get off??? In 1 min 30 secs? Come on...