ARENA I is by far the easiest of them all. As enemies are quite low in numbers and also weak, you will not amass a lot of money. The big bucks usually come around wave 7. I’ve been using secondary fire mode of the pistol all the way to wave 7.
I recommend that you build some basic defense including a turret or two as you can easily get swarmed unexpectedly. Around wave 7, I recommend the grenade launcher (7000 $) with its secondary fire mode. As the video shows, I picked the flame thrower which made the final boss fight a game of patience.