You need to play rounds of golf 9 or 18 holes at a time (regular stroke play or tournaments) to increase your rank the fastest. Lowering you average score is also necessary and the only thing you can do that is to play 9 or 18 holes of regular golf. CTTH, Blitz and Alternate Shot golf will increase your experience score, but will do nothing to lower your average.
You can also use the "boost drinks" if you want to pay to increase your levels. In my opinion boost drinks is just another way for WGT to make money. Pay to increase levels, what a concept!
Hope this helps.
1goodgolfer
P.S. Play the easiest course (for you) and the average score will come down faster as you improve your game.