On top of Death Ward, Spell Immunity (and it's variations) can make a target completely immune to the effects of Enervation. Note that it only affects 4th level spells and lower only (8th and lower for the greater version). So it should be available as soon as Enervation, but it's not available for arcane casters.
A Black Soul Shard can absorb a single negative level.
And a creature with Negative Energy Affinity will be healed by Enervation as if it were an undead creature. An Umbral Dragon is completely immune to Enervation. Vemleks are a low CR (3) demons that is also immune to it's effects. A 4th level Agent of the Grave can obtain *Negative Level Affinity aswell.
A Damphir character will also behave differently against Enervation. They will suffer no penalties from it, and will only die if they have enough negative levels as their HD, after the spell's duration is over, all negative levels are lost.
However, remember that the Thanatopic Spell Metamagic could allow you to cast Enervation on targets that are resistant/immune to it.