Ryse: Son of Rome

This sounds tougher than it is. Select the map Forbidden Forest from the survival part of the multiplayer menu, pick Diana as your god and start playing with your partner. 

We are going to do two things here: normal environmental executions, and use our focus hurricane to push enemies into the logs. Upon entering this arena you will see a few pits where environmental executions are possible, but you will notice that most of the arena walls are lined with log spikes. It is upon these walls you will try to do your executions/hurricane. 

Method One: 
When they are weak and they can be executed, make sure there is a 'cracked skull' icon above their heads. This cracked skull indicates an environmental execution. If there isn't a cracked skull icon but you are infront of a wall, do not execute the enemy but try to position your body so you line up directly with him and the wall. Once the cracked skull icon is present, hit your execution. I only noticed one execution animation against these walls and it was a double . 

Method Two:
In conjunction with method one, use your hurricane focus ability to push enemies into the spikes. Each kill like this also counts as an environmental execution and speeds the process up no end.

Simply repeat on thirty enemies.

You can also use environmental executions on other features in the map, such as the pillars or pits. However, the log spike method is by far the easiest and fastest. The achievement is cumulative, so it doesn't have to be done in one round. The achievement will pop mid round.