Here is the full list of all 10 Law Mower achievements.
Kill an NPC mower by having them run into a gopher in the campaign.
Kill 5 dogs with your mower within 1 second in the campaign.
Kill 5 gophers simultaneously with a rocket in the campaign.
Kill an NPC mower on Cruel & Unusual difficulty.
Kill two birds with one rocket in the campaign.
Kill two NPC mowers by making them collide in the campaign.
Complete the campaign on Executioner difficulty.
Complete the campaign on Executioner with 3 stars.
Complete the campaign on Cruel & Unusual difficulty.
Complete the campaign on Cruel & Unusual with 3 stars.