If you want to practice of basic techniques: aiming and moving, dodging, and familiarity with weapons, try the "Gael" deathmatch level set with 16 players. Use arena mode to practice using and defending against a specific weapon.
Just for kicks, though, I like to play Redeemer arena with 16 players in Gael. The lights are so pretty.
Today, being Thanksgiving, I've been playing Instagib. Just seems appropriate.