The best way to construct a frontier team:
1. Physical attacker
2. Special attacker
3. Tank/Mixed
The team that I got all the silver symbols with (excluding the factory):
Milotic (Tank)
Metagross (Physical)
Latias (Special)
Try and limit your weaknesses, Snorlax and Slaking are 2 good ones being normal types, and most good type ones have 2 or three, such as milotic and Metagross.
Latias has 5 weaknesses, but she's the best (and only) Sp. Attacker that I'll ever have, besides Latios.
Pokemon that can put Leftovers and Shell Bell to full use are also good, any pokemon that can learn recover is good too. Rest is also an option. An exploder can be used as a last ditch attempt
Beware of Shedinja
This annoying bug would've killed me if not for my milotic knowing recover (it killed itself using struggle)
Greta's shedinja lost on judging, so that was an easier battle.