Easy achievement. Best way is Create a Private Session, make it easy, make it invite only...then just play till the Zombie slots engage (round 5).
Once the slots stop rolling and you get a boss just kill it. If you don't get the boss or bosses that you need just quit and try again.
When you kill the boss and get the achievement just quit the game and do all this again for the remaining Garden Ops boss achievements.
- Using the Peashooter is the easiest
- Use your Pea Gatling and Chili Bean Bomb
- Have a Sunflower in a pot nearby in case you get hit and need a quick fix of health
The bosses that you need for achievements are the Gargantuan, the Disco Zombie and the Yeti.