Ok , first of all this will only increase your ladder points . The global rank doesn't change too much . Also I think it's better to try from lvl 10 to 15 , after 15 you'll get some crappy points ...
Ok now :
1) Go to Songs
2) Scroll down and find Play Him Off, Keyboard Cat by TeeKicks ( it's no 5 atm )
3) Play Guitar -> Tap/Strum -> Skilled + -> Vs Computer
4) Finish the song 100% ( not very hard )
5) Do this as many times you want
The song is only 0:24 long .
Here are some SS :
Canon Rock (one of the most complex song in the site) gives you 4k points if you finish 100% on Normal and it's 5:25 . So you can get the points for that in two "Play him off , keyboard cat" plays which will only take you 0:24+0:24 = 0:48 .Now let's calculate how many points you can do in 5:25 with the 0:24 song :
hmm I suck at maths lol but I think it's @ 21 plays of 0:24 and 21 x 2.000 = 42.000 ladder points .
Have Phun , click thanks if you enjoy it