I have problem with level up system with this game as many players , too .
My Suggestions for level up system .
1. do not restrick how many heroes that can be level up .
I think this is best choice of GLU . for players just play other heroes and wait , with some-boring .
2. auto level up every heroes and keep extra-EXP to continue next level up , player can spend 5 glu coin for extra skill point from previous level up .
I think this is best choice of player . player can continue playing with out any boring .
!!! It not fair that my heroes losing extra-EXP !!!
3. simple level up with out any waiting time .