Route 221
go to the pal park then go near the berries and there will be a trainer (ace trainer shannon) and fight her and she will have a lvl 30 cherrim and 2 other Pokemon
in Pokemon diamond go to iron island and find a camper trainer and fight her
Fight every trainer you see if you can. Many trainers hanging around even normal places will have that rare finneon or cherrim.
you battle a Trainer and the trainer will say before you fight him EVERYONE
fight every trainer or TRAVEL that's what your journey is about travling to new places and discovering Pokemon
A trainer with a cherim can be found in the iron island complex Someone previously mentioned Eterna Gym but even if this is true it isn't a very usefull location for those looking to add the Pokemon to their pokedex to get the national dex because gym trainers won't fight after their leader is beaten.
If by strong you mean higher level, the stronger trainer is your rival, which you can battle at the weekends at the Fight Area.
as a good and experienced Pokemon trainer, I would say a well balanced team at about 56 to about 64 with several different types.
In snowpoint city at the bottom there is a sailor that recognizes you as a good trainer and takes you to the fight area after you beat the elite 4
the gymleader that specalizes in fight
cant fight just capture
you get to fight ash, but he apears as pokemon trainer red.