To get all achievements/upgrades: leave 1 zombie alive, and collect all the boxes and resources. Boxes at the far left area give the most gold. After boxes stop spawning, kill the last zombie.
Can beat all game with only Light Cavalry. Only buy relevant upgrades (from top to bottom, best lines are 4th, 3rd and 1st, 2nd only has range unit upgrades). Quick wins - rush a single lane with light cavalry, you'll get the enemy to 0hp before his units reach your base. Then just finish it off.