Currently running Ephraim, Julia, Takumi, and a low level Olivia to Dance Dance Revolution them. Had a near perfect run in arena bringing me to 4153 points, but I haven't been able to pull off a full no-death run. The map with the 2-2 split of units is very hard to keep Olivia alive on.
No death runs on Advanced aren't too hard if you cheeze the system a little. I had none of the focus heroes for this season, except for Narcian, so I put in a level 7 two-star Narcian and a bunch of 40s. It just matches the overall stats of your units, so it was putting me up against level 25-30 parties. My 23 speed Cecilia was running around doubling everyone; it was pretty great.
It only gets worse the higher you go, my highest units are in the low 30s and I can easily go through my entire stamina bar without gaining a single level on any of them because of shitty tower maps causing outright wipes or just killing the characters gaining most of the EXP each time.
Mmm...not that I haven't wiped in the training tower, cause I totally have. But like...the only unit I abandoned levelling cause he died too much was a 3-star Odin whom I was only levelling for the Arena focus hero bonus, before I decided to stop being cheap and promote my 2 star Narcian to 3 star. But my 2 star Anna is about the same level Odin was and is doing fine.
In general the lower the star level the character the harder it will be to level, of course, since you get a lot more exp when killing enemies higher level than yourself, so having high stats for your level is helpful...which 4 star and 5 star units have. Getting 4 star heroes to 40 is pretty easy in my experience. (I didn't have much trouble getting 4 star Ovelia to 40 for instance, even though she's a dancer and "supposed" to be hard to level).
As for time investment...once I realized all my pulls were bad and I needed to promote 2* and 3* units...going 1-40 with 4-star Ovelia, 4-Star Shareena, 4-star Nino, and a flex spot (healer or a higher level unit to set up kills) took me...3 or 4 days I think? Shouldn't take too long.