If you've got a supernatural featured foe or boss that really needs to be in a scene, and one of the players is packing Banishment, then you do need to get creative.
"Soft" Counters:
- Use Illusion magic to disguise which GMCs are supernatural and which ones aren't.
- Use mundane make-up and disguises to change appearances.
- Banishment works, but the demon gets a 3-shot "Last Word" attack before the pit opens and swallows them up.
- Banishment works, but it also causes certain demons explode, inflicting collateral damage (and possibly a catastrophic "chain reaction").
- Villain chooses a scene location with juncture modifiers tweaked against sorcery.
- Enemy human sorcerer uses Bend Fate to sandbag the PC's roll.
- Boss has an artifact or macguffin that boosts his resistance, or gives him a limited number of "counterspells" before it burns out.
"Hard" Counters:
- PC has been cursed (either off-screen or otherwise) so that Banishment automatically backlashes until certain conditions are met.
- Boss has an artifact or macguffin that automatically negates Banishment. Macguffin needs to be stolen/destroyed first.
- Boss is flat-out immune to Banishment unless the PCs can locate an ancient scroll or some other macguffin.
- Scene location is hallowed/unhallowed ground and explicitly doesn't allow Banishment inside a certain area.
- Boss has clones or duplicate bodies that he can activate or re-occupy quickly.
- Boss has a "revolving door" policy with the Underworld and can come-and-go as he pleases. If banished, he just shows up again in the next sequence.
- Boss pulls out a card that says, "I have Plot Immunity", shows it to the PCs, puts it back into his jacket, then continues to fight.
(In the last example, I actually had a card printed out and physically handed it across the table to a player. He even got to keep it, and I suggested he could use it later in another scene to get his own character out of a jam. The players absolutely loved this.)