L2 is a term used in a phrase before any noun. It stands for "Learn To" and is deliberately improper grammar in 1337SPEAK fashion. L2 (Noun) is used as an insult, telling someone to attain a certain skill or learn to properly use a specified noun.
Gamer1: WTF?! I SHOT AT HIM 10 TIMES AND DIDN'T KILL HIM!!!
Gamer2: Lulz, nub. L2 headshot.