Vertix.io is one of the first shooter .io games to be released. This game is awesome. You control a player on a 2d platform where you battle against a bunch of other players.
All you have to do is use your WASD keys to move around, and use your left mouse button to shoot! You can switch weapons with the scroll wheel too.
The best strategy in Vertix is to make sure to always keep moving your cursor back and forth around the edges of the screen. When you keep your cursor near the edges, you can see farther, and get an advantage over your enemies who are not doing that. Good luck!