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