I think that either the retaliation damage should be higher or the Avenger's health should be higher. Several CC cards can easily knock it out. The point is that the opponent has three options when facing the Avenger:
A) Leave it alone; allow it to deal 7 hp's worth of damage PER TURN.
B) Destroy it; take a ONE-TIME hit of 7.
Between the above two, it's still better to target the Avenger than to leave it alone. Option C is much like B:
C) Delay/Freeze it for a few turns; take a hit of 7.
Choice C is still preferable to A because it'll take a while for the Avenger to recover and become a threat again. Also, as a late-game fighter ( with the high cost ), it may not even get out of the delay/freeze before the match is over.
Case in point: From what I'm seeing, I'm inferring that Death Avenger is meant as a high-cost, high-power creature with an ability meant to deter CC; however, the retaliation damage isn't high enough to make it worth the cost, nor is it repellent enough to make somebody think twice before killing it.