Single position is the only way to get them to that level ** in the short run. Long term two position training can be as effective.
I am also a firm supporter of single position training, but I think it comes with the caveat that you have to be willing to transfer players. When I was just starting out I bought a couple guys, trained them, sold them off in waves and bought younger/better players each time. Also use your single position players to upgrade your other positions. Train 3 PGs for a season, keep one, sell two, buy one good young replacement PG, and one good older big man.