If the goal is to minimize the game hours on your character when you get your 1st tier form then you want to minimize the time you're online but not earning exp. Or, online but earning less than an "optimal" amount of exp. While you can solo rank, its almost always going to be "slower" than ranking in a group, which will result in you having more hours on the character when you get the 1st tier form.

If the goal is to minimize the amount of RL hours before getting a final form then solo ranking is fine. Advisable, even, when a group can't be found.