Basically your logic says if ArenaWaveCounter = 1, keep creating these two enemies constantly. You need to put a trigger once condition under the ArenaWaveCounter.
Also you've got a typo on Wave 3 there that should be ArenaWaveCounter = 3 but it says 2 so you'll need to change that to make that work properly later on.
The spawning before you hit the activator could be several things. Something could be triggering ArenaWaveCounter to 1, maybe its 1 by default? It could be that your hero or a version of the hero off screen or something is overlapping an arena activator somewhere in the level.