Most of the achievements aren't very difficult to attain, and those that are difficult are more time consuming than challenging. I remember several months ago (might have been as far back as September 09) there was a conversation in an online game over what "appropriate" difficulty achievements would be, and I wrote down some of the ones that got suggested. Maybe try a few of these:
Serious Business
Defeat a human opponent in an online match.
Blitzkrieg
Defeat an unfair difficulty opponent in under an hour on the random-small map.
Volunteer Army
Defeat an unfair difficulty opponent without ever researching any fleet or capital ship crew upgrades (Sins Vanilla only; no Entrenchment)
Interstellar Sanctions
Defeat an unfair difficulty opponent without ever using the black market on the random-small map
Survivor
In a multiplayer match, lose your empire, but escape with a few ships and re-build elsewhere and rejoin the battle.
Master Economizer
In a multiplayer team match, get your income up to 300 credits per second... after upkeep.
Master Aggressor
In a multiplayer team match, maintain the largest fleet size with the smallest economy and least amount of feed. Defeat at least one enemy by yourself without taking excessing casualties.
Master Defender
In a multiplayer team match, survive a two-front war against multiple opponents while cut off from allies. Never allow your enemies to capture any of your planets, and never lose control of your homeworld.
Master Comp-Stomper
Defeat a team of 5 or more allied (locked teams) unfair difficulty AI's on a randomly generated map.
Climatic Showdown
Fight a multiplayer battle against over 2000 command of human-controlled enemy units and win... without any help from your allies.
Carry the Team
In a multiplayer game of 3v3 or larger, have the largest empire, strongest economy, and largest fleet of anyone on your team, all at the same time. You must go on to win the match.
First Pick
In a 5v5 multiplayer pick up game, be the first player to be picked by the team captains.