HEY YOUR FIRST FREE MENT!~
*This is an unmissable achievement You can get this in the start of the game; you'll be prompted to take a gun from Claptrap's cabinet in Claptrap's Base.
For tips I'd pick Siren she's by far the best for this game. She has an ability to save team mates later in the game when you level her up. If you are playing co-op this helps out alot and she can regenerate health faster than other characters. As well as phaselock which picks up enemies’ that hurl in your way. Trust me there are TONZ of enemies’. Have Fun! :D
You'll also get the next achievement with the help from this video guide;