1.Make the main menu better. Show that easy means difficulty so that it's obvious. 2.have different maps for each wave or have this thing to create a random map. 3.Health over enemies. 4.also make a better tutorial. 5. There should be "ickups" such as health and things like that. 6.Make it so enemies don't follow you!!! have them use some strategy by calculating stuff.(if user is here, AI shoots here) etc.
As already brought up,the AI looks like a fan of your guy(they're always following you).I think that 2 things that would make it better are: 1 different weapons 2 Adding allies in a manner like "for 3 enemyes,one ally is added.
Good start for an arena game.Played till round 10.