Here is the full list of all 18 Walkover achievements.
Get 100000 Score Points.
Build 100 Towers
Collect 100 weapons
Get 1000 Score Points. Lame but it's a start :D
Kill 1000 Bugs
Get 1 Million Score Points.
Kill 25000 Bugs
Kill 100000 Bugs
Kill 500 Breeder Bugs
Kill 10000 Breeder Bugs
Kill 50000 Breeder Bugs
Build 1000 Towers
Build 10000 Towers
Call 100 reinforcements
Call 1000 reinforcements
Call 10000 reinforcements
Collect 5000 weapons
Collect 50000 weapons