Absolutely this.
It may sound stupid to use xp potions on a lvl1 champ to get them to like lvl10-15, and then just sacrifice them anyway, but you get tons of points this way, and you dont expend any energy.
When theres no champion training events around, you should be getting 3 star and 4 star champs maxed out ready to by ranked up. (alongside whatever else you are working on. minotaur, iron twins etc)
Basically, you're prepping so that when the tournament starts, you've got a bunch of lvl1 champs (rank 3 and 4), and a bunch of max-lvl rank 3 and 4 champs. Give potions, rank up. Ideally you'll end up with a bunch of lvl1 rank 5 champs. Give them potions too, and then upgrade champs you actually want to use, to rank 6.
I could easily do 20k points in the first 30 minutes, including ranking up 8 champions to 6 stars, if I want to. Or save them for next time. Whatever suits.