We are approaching the final session of our 3 year long campaign. My players are level 18 and about to enter the castle of necromancer that will end in a battle with Orcus. Leading up to this I have several puzzles and encounters for my players to deal with and I'm worried they might not be strong enough to face Orcus by the end of it all. I had considered a moment where a god one of the players worships steps in and grants them temporary aid. Originally I thought I would bump them to level 20 for 1 minute during the fight against Orcus but don't know if mechanically it will be enough. The other thought was to give them the effects of a long rest but this feels a little too deus ex machina.