Defense Grid: The Awakening

Get all of the gold medals needed for the "Alien Tears" achievement, then go back and play the "Relaxed", "Practice", "10/20/40k Resources", and "10 Tower Limit" challenges for each stage. Those are insanely simple and will count towards the achievement.

This achievement is easier than it sounds, for anyone who is simply gleaning difficulty from the list.

The Awakening campaign (the one that ships with the game) contains 149 total gold medals. DLC medals count for this achievement as well, so the Resurgence campaign adds 40 medals and the You Monster campaign adds 42 medals for a total of 231 available medals. Remember that you only need 40 medals for this achievement, so just cherry picking some easy modes will get this achievement out of the way with little effort.

Average difficulty
Story: Standard campaign mission.
Frozen core: Can be easy or hard depending on the map. Stolen cores do not return back to the housing unit, so baton passing can become a problem.
10/15/20 tower limit: Easy up until level 8, then become pretty much infuriating.
Story reversed: Standard campaign mission, but all exits and entrances are swapped. I have never played one of these, but just like a regular story mission it varies based on the map and requires trial and error to defeat.

Easy modes
Fully loaded: Standard campaign mission, except all towers and upgrades are available. These are only available early on because the tower upgrades unlock as you play, which basically means the game is letting you bring guns to a knife fight by plopping maxed turrets into early missions.
Relaxed: The enemies are weaker, and you sometimes begin the level with extra resources.
Poison core: Baton passing is a complete nonissue because the dropped core instagibs all enemies in its path as it returns to the housing. The enemies killed by the poison core do not contribute to your score so this mode has propensity for failure if significant groups of enemies get gibbed, though I never once failed one of these.
10k/20k resource challenge: Story challenge mode, but you have a fixed amount of resources at the start and never gain more. These modes are a nonissue though, since a strategy that works for the story challenge can be replicated directly at the start of the level, then you can just put your feet up and watch YouTube until you win.
Practice: You start with 20k resources at the start and never gain more. Similar to the above in that a winning strategy can be deployed directly at level start, but the difficulty is set to normal instead of hard.

Hard modes:
Story challenge: The enemies are tougher, and in some cases you start with reduced resources.
Out of bullets: Standard campaign mission, but you cannot use gun towers or cannon towers. I decided not to play these because I did not feel like structuring another winning strategy for each level to work around the contrived limitation.
Balanced build: Standard campaign mission, but you must build one of each type of tower before you can build another of any type. This is another mode I never tried for the same reason as above.
Green towers only: Standard campaign mission, but upgrading is not allowed. Another mode I never tried for the same reason as above.
Shredder: 99 waves of assorted enemies. I tried this mode once and never played it again because it just keeps going on and on for what seems like an eternity. Far too much time investment for a single gold medal.
Grinder: 99 waves of the standard weak enemy. Same problem as above.
Super grinder: Never played it, but I would presume enemies are more difficult and/or more numerous. Likely has the same problem as above.

With all of that in mind, you can easily pick up this achievement by level 10, Veil of Ice. My recommendation is to perfect (or find on YouTube) a strategy that works on the Story challenge mode, then use that same strategy in Story, Relaxed, Fully loaded, Frozen core, Poison core, 10k/20k resource challenge, and Practice modes. This way, you can get the achievement halfway through the game.

If you have no stomach for that much repetition, you can simply choose one challenge from each mission to complete this achievement as well. Look for easy modes on each mission, or at least a mode of similar difficulty (e.g. if there is no Practice or Relaxed mode, go for Fully loaded or Poison core because the difficulty is the same as the regular mission).

Not so much a solution as some usefull info.

The map packs count towards this. For efficency and the fact you need gold medals on them anyway, do the AWAKENING story missions and work your way through the RESURGENCE maps. Finally complete YOU MONSTER story.

If you only have some of the DLC do the story missions then pick some of the level variants to mop up.

Awakened story = 20
Resurgence maps = 12 (will check exact number)
You monster story = 8

Along the way you will get:

All awakened story achievements.
All Resurgence achievements
Tower expert achievement
Exterminator achievement
3x you monster story achievements.

If you follow the video guides for ALIEN TEARS, the resugence achievements and the YOU MONSTER achievement you will be fine.