Here is the full list of all 20 Mowin' & Throwin' achievements.
Throw a rock at another gnome and stun it
Pickup two gas cans while boosting
Stun your teammate by throwing an item at them
Mow 100% of your lawn before time runs out
Boost the mower off the edge of a lawn
Get a tie game
Cause a gnome to be the victim of a 6 second stun lock
Get stunned by a falling care package
Dig up a fertilizer that is less than a second away from exploding
Leave the game running on the main menu for 2 minutes
Play a game where a gnome receives a gold end game trophy in all categories
Scroll through the entire How to Play menu
Knock a fertilizer out of a gnome's hand by throwing a fertilizer at them
Have three thrown fertilizers simultaneously counting down on your rival's yard
"Accidently" push your teammate off the edge of a lawn with the mower
Come back and win a match after being behind by 20%
Win in the last 5 seconds of a match after being behind
Knock a gnome off of a level by hitting them with an item
Walk off the edge of a lawn while affected by the Hypno Shroom spores
Finish a match on every level in the game