For medals you should check via matchmaking people that have enough medals and good enough that you can beat those bases and save them in your favourite list so during league raid them (3 times only every 1h) for example i save sometimes people with over 400 medals 450 medals then it becomes hard for me.
Then to get high medals from enemies you should stay also at top leaderboard, there is also a system called Medal Bonus that allow you to gain more medals (only via matchmaking) depending on what rank you are:
rank: 1-10 medal bonus: 50%
rank: 11-100 medal bonus: 25%
rank: 101-500 medal bonus: 10%
rank: 501-1000 medal bonus: 5%
rank: 1000+ medal bonus: 0%
If you are in this placement you can gain more medals during raid if you do 100% on them and only via matchmaking as i said before.
Also even without medal bonus if you are in top top top you can get enough medals to win leagues also without medal bonus. ^^