For the Legend achievement, yes it has to be on heroic, but no, it doesn't matter if you lose a fight. You'll still get the achievement. Also, http://armorgames.com/community/thread/3572605/guide-all-achievements-one-game
Did something get changed? I beat racetrack a good dozen or so times with one unit getting all but a couple kills, and now I'm getting stomped on that map...
WAY too easy. Enemy damage could increase by an order of magnitude. Also, beating it on seizure mode should be worth a badge. That takes some dedication...
Did level 9 without too much trouble with a modified design that couldn't hold its own weight that suddenly held a safe. This physics engine is really cruddy, on level 10 I'd modify one bridge and the other would break... I finally got the first bridge to not break by replacing out one 1-meter-long piece of steel in the SECOND bridge with wood. I kid you not.
I made a popsicle stick bridge that held over 800 lbs. It weighed 12 oz. And I'm stuck on level 9. This can be fun, but like Cantak said, it doesn't have much to do with actual physics.