u/EatenFisher

I need help how to make a boss fight where the focus is sealing the boss back while still engaging.

In context, the party is attacking the boss' lair that has been terrorizing around the region, the party got the key that can be used to unlock or seal, and they discovered this was already used to unlock the prison the boss was in, as the boss is an alien demon kind of thing, the party has no power to kill it yet, and all they can do is seal the boss back into its prison.

I said that but I haven't make anything in regard to how the fight will go, I appreciate any suggestion!

In my current campaign, there are several figures that may try to stop the party, some of them being a duo of human that are trained to fight enemies with magic, and I am clueless how to make their statblocks along with their abilities in a way that is not going to be unfair to the party since the party has two spellcasters but still want to make them a great challenge. Any advice is appreciated!