Get Off My Lawn was coined in 2008 by Clint Eastwood, regardless of what you think or have read. His character "Walt" uses it to warn a bunch of Hmong kids to gtfo his lawn. It is said that the actors all soile themselves in each take of this scene.
Get on My Level is used primarily as a form of trash talk, more commonly in online gaming. The user tells the recipent that he is basically not worth his/her time, and is lower than semen dispersed from a used anus, so he/she must "get on his/her level."