playing only 2.5 seasons, what i ve learn is that you train big men for example (my team), and eventually you sell 1 of the trainees, just to save money to upgrade the positions that you dont train. I dont like the mono skill training too. I train 1 skill until all of my trainees get a pop up, and then i train an other skill, with that way i have a very balanced big men, that they dont do anything spectacular but lots of things very good. About the arena, if you dont need a good player for now to be competitive spend money on the expansion, but try to be balanced. I am upgrading my arena as soon as i see that the sits are full on a very high price.