Know the routes, so you don't end up wasting coins on recruiting. Double magic with level 7s (especially red) is good early on in Secret Quest, because it will OHKO everything including enemies hiding behind dual coloured shields. But later if you're no longer OHKOing monsters with the magic, it's better to just do a group attack because of the critical chance, even if the magic output is better (ie; 8,8,8,8 vs 40, I'd rather take the chance on the crit).
For the final fights, where you're just hacking away. Bringing two level 7s per usual and two wandering Miis is great; group attack with the level 7s and then potion of valour to bring them back with one of the wandering Miis. Group attack again, then potion of valour with the other wandering Mii to bring all three of the previous back. You don't want more than two wandering heroes, or you could accidently bring back a wandering hero over your level 7. You can do this as long as you can afford it. And it's far cheaper than buying level 7 Miis.
Thankfully, we're a three 3DS household and Toronto is a great spot for passing too. So I don't really worry about spending coins.