This is quite simple.
Select Missions > Battles > Solo
Then choose any battle mission, the hero battles may or may not unlock the achievement, it didn't for me, but it has for others, so try at your own risk.
Any of these missions are going to be rather straight-forward, but I like Endor. It's a smaller map and everyone convenes in the middle for a nice little battle.
First to 100 Points wins.
Scoring breakdown goes as follows:
Standard Soldiers: 2 Points
Snipers: 2 Points
Jetpack enemies: 5 points
Heavy Gunners: 5 points
You must pick up the orb that enemies drop in order for the points to count. They disappear after 12-15 seconds.
I hope this helps, let me know if I can improve anything.
Here is a youtube solution provided by SilentRich69