Depends on how you play.
If you have alot of time on your hands and not multitasking, 2-7 is probably the best way to go.
If you need to do something else while playing (homework etc), do the highest leveled 5 ally deploy quest you have. Since it is 5 minute cooldown, it will take longer, but you can make up some lost time by doing training whenever you can.
What I do: Quest to level 30. Shouldn't take too long. Feed 20 lv4 2* to it to make it lv40. With the bonus, you only need 13 to bring it up to lv40.