Is there an unobtainable achiev in the game?
Nope! One was flagged as bugged, but it'a obtainable anyhow.
How was this DLC?
It's easy enough; it just takes some time to re-adapt to the game's controls and learning the AI's behavior. For 5$, it adds a reasonsable amount of TA for this title.
Score over 250 kills in Team Death Match - Prey (Xbox 360)
Play ten Ranked Multiplayer Matches - Prey (Xbox 360)
Score over 250 kills in Death Match - Prey (Xbox 360)