Hiya!
Wizard or Lore Bard. Nobody else can come close to Contingency Banishment(Self) to buy them time and do horrible things to their foes.
A "Contingency -> Banishment (Self)"...not gonna work.
Banishment is Concentration up to one minute. When a creature is Banished...."if the creature is native to the plane you are on, you banish the target to a harmless demiplane. While there,
the target is incapacitated. " Looking at Incapacitated, "Can't take actions or reactions". Looking at Concentration, one of the stipulations is that you can't be dead or Incapacitated.
So, your Contingency goes off, you go to a demiplane where you immediately become Incapacitated, breaking your Concentration, and immediately popping you right back to where you were...minus a 4th level and 6th level spell.

I'm sure your opponent appreciates it thought!
Personally, I went with Cleric. Being able to say "Odin, I need serious help here"...and having Odin hear and actually DO something to seriously help you is pretty much an "I win" button in this fictitious white-room scenario. Whatever/Whoever is your opponent is roughly equal in power to you (the cleric), is prepared with the specific intention of killing you. Pretty sure most Deities don't want their most honoured and faithful priests to get killed in a round or two...that's just bad PR all around! (As a meta-game...the Deity in question is the Dungeon Master; the Cleric just asked the DM to help him win...

).
^_^
Paul L. Ming