I have been thinking about ways to stop this buddy system and have come to the conclusion it is impossible if you can set a defense team, the buddy system will allow you to gain the top ranks.
Some suggested solutions and why they won't stop it
Make it cost crystals to change defense team. That is just a pay wall and the one's at the top using the buddy system are already willing to spend
Put a time on changing defense team or make defense team a one time set and done thing. If you can set a defense team it can be explioted in one way shape or form. Currently it is the 2 man buddy system where defense is changed back after a bit to a harder team.
So lets say you go with the extreme option of only allowed to set defense one time. You now go to the 6 man buddy system, this is assuming rank 2 can only be seen by rank 1-6.
How it will work:
6 players get on the same leaderboard
2 set really easy defense teams the others set harder defense teams.
The 6 work together to get to the top and keep the 2 easy defense teams in ranks 1 and 2. Now the only players that can hit the easy defense team is player 1-6