This was the finale of a year long campaign. The party were fighting an evil angel (the BBEG), a cleric who had been tricked by the angel and a king who had been manipulated by the cleric. The party was level 10 and the cleric and the king were tough but definitely bearable. The angel was brutal, doing 5d12 with three attacks, along with legendary resistance and actions. It also has a magical machine on the room it can use to heal itself as a legendary action.
The party early on realised that they could only beat the BBEG if they dealt with the two mini-bosses first so they tried to banish him. They burned up his legendary resistance with a few other spells and then attempted the banish. The cleric succeeded on banishing him so naturally the two mini-bosses try to attack the cleric to break their concentration.
So naturally the paladin banished the cleric. Well... The paladin has an AC 21 and has a magic item that allows them to do damage to an enemy every time they hit him. He also has war caster and every concentration spell is rolled with advantage.
Well... They then easily defeat the two mini-bosses and then destroy the machine used to heal the angel.
This was a really funny moment and I think the party saw the defeat in my eyes when I realised I wasn't getting my angel back for 10 rounds.
A question for DM's, how would you stop this if you wanted to avoid the party banishing itself like this?