Maining A Champion
Countering is not for low ELO, 99% of the time.
I'd be more afraid of the person who has mastered fighting with a broomstick, than a mediocre swordsman.
Timing! If you main a champion, you know the precise animation times for each ability. Allowing you to better react to situations.
No matter who, each champion has a few unfavorable match ups. You should off-main at least 1 champion that counters your Main's counters, for when you are picking into them. "the enemy of my enemy is my friend" though you would also not be wrong to stick with your main because your experience may outweigh the counter.
Choosing a main is simple. Go to the website op.gg, look at your win rates across all modes. You should probably be maining your highest win rate champion, given the percentage is based on a decent number of games.
does not counter my (S4 Main)
you Bronzies.