Most useful advise here I could give about privilege escalation is to enumerate the service everyone’s referring to and look at some youtube clips regarding b** h******* to get familiar with the commands and v***** interface. Also it took me days to figure out that nc can’t simulate an *** daemon, so you’ll have to set up one yourself or do manual responses with nc like someone already advised, personally I used a python script I found on github. A tough box, but learned lots of stuff about networking! Next step is to find a decent book to get a bit more familiar with this stuff, any recommendations someone might have would be welcome.
All I can say is if you believe you know the privilege escalation attack vector and sure of it keep trying, you’ll get to it eventually.
Also look at s4rgey’s comment.