Fun concept and neat execution. I did crash the game once with a "Variable greenish.trip_timer(Lots of numbers, lots of numbers) not set before reading it. at gml_Object_greenish_Step_0", by tripping an enemy before it had spawned, but other than that, works well.
It also has the thing where running diagonally is faster than running cardinally. Combined with the arena, I believe that the strategy would be to place each bloak in opposite corners, controlling the bloak on the side the enemy spawns in, running diagonally around the enemy, and defeating them before another spawns. This is much easier said than done.
All in all, Great Work! I could see someone getting very good at playing this.
High Score: 250