First: The traitors will always be the same. I personally only had one traitor, so I had a bit more to work with.
Second: How you come out of the traitor fight is important; if your turn is just about to end when you finish the last traitor, then you won't be able to act in the first turn of The End battle. This can also work in your favor, all buffs you equip during the traitor fight carry over into The End's fight, so buff up. You shouldn't need to waste much sanity on the traitor's, they don't have a huge amount of HP(about double their normal) so they shouldn't be too hard to bring down. Basic attacks should suffice.
Finally for The End boss fight: The End has 3 moves. His basic attack, a powerful AOE attack, and his Rage clone explosion attack.
The basic attack has as much range as anything you can throw at him, so every time you hit him, he'll counter for a load of damage and he has a high crit rate. If you have Nagi's Dive skill equipped on someone, you can use it to start an assist chain with no drawback. Before that, get all your people in position and use buffs while doing so, keep them far behind The End. Don't group up too much or he'll bomb you with his AOE.
Once you do enough damage, he'll have a monologue. Once that happens he's about to use Stream of Rage, which clones him 3 times. Only the clones can attack, and they'll only use basic attacks, The End himself will just stand still and do nothing. Two options here: stay the fuck away and wait for them to dissappear in 3 turns, keeping in mind that you need to hide behind the barriers around the map to avoid getting nuked or you can kill them, which will greatly reduce his sanity to the point of stunning him.