Play Solo Arenamode and choose your favorite Arena. As God choose
Mars. The Damage AOE from this God makes the Achievement quite easier.
The Wave with the Bunkers is random in any Arena. The Key for the Achievement is,
dont destroy the Bunkers. The Enemies are infinite at this Point. The Enemies which come out from the Bunkers
died with 2 Hits.When the Wave starts, kill as first the 3 normal Enemies in this Wave, in my Video as example the 3 fat Guys with Shields. After this, simple kill the 25 Enemies. The Kills from Focus / God counts too.
PS: You can do this too, at the "Kill the Archers" Wave in Solo Arenamode. After the first big Wave of Enemies, it comes only 2 Enemies until you kill the 4 Archers. Hide for the Arrows from the Archers and kill the Enemies which comes to you.