Pretty Simple..
I Think You Should Put It At L2PcInstance, search for "setPvpKills(getPvpKills() + 1);" And Add The Code After It
( That Way When Player Gets A PvP It Will Check If He Just Got Enough PvPs And Change His Title.. You Could Also Add It To Enterworld, If A Player Logs In And Has Changed His Title It Would Change It Back To His "Rank")
This Post Would Be A Bit More Interesting If You Removed The Players' Option To Change Their Title(Dump Idea, But Nice In Case You Cant Do What "An4rchy" Said :P..)