One Small Step
Complete one level.
You'll get this after completing the 1st level.

Boom Baby
Kill 2 enemies with a single Bomb.
This is very easy to achieve with the Bomb or the Smart bomb skill.

Disciple
Clear the Training Area.
You'll get this after completing the 2nd level.

Look Mom I Did It!
Kill a Big Walker.
This achievement is awarded when you kill you're first Big Walker. The first big walker appears in level ?, I forgot, you'll get it, don't worry.

Monsterkill
Kill 10 enemies with a single shot
This can be very easily done with the Smart Bomb skill. Wait until 10 weak enemies are on screen and use the skill. The achievement will be yours.

Tower Master
Complete a level without using any skills.
This is easily done on the 1st level, especially when you have unlocked the stronger towers.

Core Guardian
Get 100 perfect rounds.
A perfect round is when you have no core damage. Do this 100 times and you'll have this achievement to show for it. Cool!

Environmental Expert
Clear the Facility Compound
You'll get this by playing through the levels.

Bring It On
Beat a level on Hard.
You can set the difficulty before you start a level.

Electrifying
Hit 10 enemies with Thunderbolt.
Get this after you have unlocked the Thunderbolt skill.

I Hate Maize
Have no blocks at the end of a level.
You can get this easily in the 1st level Academy, Just complete the 4th round without any blocks (sell them after round 3), so only use skills in the last round.

Now We're Talkin'
Unlock all tower and skill slots.
As you play, you earn XP, as you earn XP, you unlock towers and skill slots.

Golden Idol
Kill the golden Walker.
This achievement is based on chance. Every once in a while a golden walker will spawn and when you kill it you'll get the achievement. When will it spawn? It might be the next 1 and it might take another 10k walkers. My guess is it spawns about every 3-5k walkers. You will need to kill 10k walkers anyway for the Liberator achievement so you'll probably have it by then. The best level to farm walkers is on level 3 Gallery in Survival mode.
Check my video in which i do NOT get the Golden Idol, missed that :(, just to show how I farmed and got it: http://youtu.be/sdDj7zsFu_8

Aerial Ace
Clear the Hydro Islands.
You'll get this by playing through the levels.

Skill Master
Beat a wave with only skills and blocks.
You can do this easily on lv1.

I Hold Everything!
Unlock all towers and skills.
As you play, you earn XP, as you earn XP, you unlock towers and skills. Before a level, when you select you're towers and skills, you can see you're progress.

Ruin Ranger
Clear Brightholme Industries.
You'll get this by playing through the levels.

He Wasn't So Tough
Kill a Huge Walker.
You'll get this by playing through the levels.

Exalted Elysion
Clear Elysion
You'll get this by playing through the levels.

Liberator
Kill 10.000 Walkers.
The best level to farm walkers is on level 3 Gallery Survival. Level 3 gives loads of walkers. Or try to beat online scores while you're at it, like mine:nr1 on Crosshairs oh yeah!

It's Fast, but I'm Faster
Kill the Charger and all its clones.
Level boss.

Alpha as Fudge
Kill the Patriarch.
This is the last boss in the last level Frenzy.

Done and Done
Beat all levels on Normal.
Beat all levels on Normal :)

Put It Down
Kill a Walker with an Anti Air Tower.
You can elevate walkers with the Gravity shift skill. Once their in the air the Anti-air towers will shoot at them. The moment you kill one this way you'll own this achievement.
Check my video: http://youtu.be/plZvlDss9xA

Line Em' Up
Kill 10 enemies in a single Penetrator shot.
Get this after you have unlocked the Penetrator tower. Best level for this is level 3 Gallery. You can do it on the first wave. Set up a Penetrator so it has a nice straight line of sight. Delay the walkers with skills: black hole, gravity shift & freeze) so they arrive in a group of minimum 10 in the range of the penetrator. You get black hole at XPlvl 14.
Check my video: http://youtu.be/P7WrUpcGKS4

Hero of Elysion
Beat all levels on Hard.
You have to beat all levels on hard (you can tell by the yellow star in the world map).This one didn't unlock for me right away. I had to restart the level and let my core die, then i got the achievement.

Seriously?
Get all stars in all levels on Hard difficulty.
You do NOT have to do this on hard difficulty!!! I got the achievement when: 1) i beat all levels on normal difficulty without any core damage (3 stars) and 2) i beat all levels on hard with core damage and 3) i restarted the game and 4) i restarted a level and let my core die. Then it popped.