I'm raising heaps of Pokemon for the Frontier, but haven't spent nearly as much time on them. Level 100 isn't my target... I'll take the challenge at no higher than level 80 - probably at level 60, in fact. At that level, you only need a fraction of the EXP points, but your opponents won't have higher levels than you on Open Level. (At level 50, you won't always have achieved full evolution.)
When I train, I bring along my lv93
for its Pick-up ability which gives me HP Ups, Proteins and Rare Candies (and occasionally, you can get Earthquake TMs with a Pick-up Pokemon of over level 90). That makes training much easier once you've achieved the EVs you want.
I challenged the Battle Factory first, to get an idea of what kind of Pokemon combinations and movesets are effective in basic battle. I gained a Silver Symbol there after around 50 hours of effort. That knowledge is the foundation for the plans for the teams I'll use in the other facilities.