Participating in the clan league war is the only way to get league medals in CoC for free. Clash of Clans introduces clan league war at least once a month, long-lasting for 7 days. To participate in this Special war, your clan must have at least 15 members.
In the clan war league, the leader or co-leader selects the best players for the war, and the selected participant can attack in one base for 3 stars. So, it is teamwork, as the more stars your team obtain, the more league medals you will get after the war. Also, if you want more, you can ask your leader to distribute bonus league medals.
The league medal is calculated based on the stars your team obtain and the level of your clan. For instance, if your clan level is 11 and win the league war all 7 days, then you will get about 214-250 league medals, excluding bonus.
In conclusion, there is no other way to earn free league medals in Clash of clans except by playing and winning the league war.
What is the use of League Medals?
You can use league medals to purchase magic items for free in COC. You can exchange your league medals with gold, elixir, potions, wall ring, Hammer of fighting, Hammer of building, hammers of spells, hammers of heroes and some legendary statues.
I think the best use of League medals is purchasing wall rings and hammer of heroes. The wall rings are the best item to upgrade the walls in the coc dark mode.
As you know, the dark mode has limited resources, and the player must wait for 24 hours for a resource attack. Therefore, relying on elixir and gold to upgrade the wall in the dark mode is futility.
Moreover, the hammer of heroes can upgrade any heroes instantly in COC without spending any resources. So, it is wise to purchase this magic item with the medals. Heroes are the best friends to push rank in Clash of clans.