This is only obtainable in post-game.
This is an additional four stages at the bottom of the Dimension Guide listed as Flashback Episode. On the one hand, the only story character you will have access to is Fenrich; on the other, you have a level 5400 Tyrant Valvatorez. Don't be fooled by the yellow health bar, this only means that he doesn't count against your deploy limit. He has a special ability that his kills aren't kills, but leaves them alive at 1 HP as an ally.
However, the last two stages leave him permanently poisoned. You can heal his poison on your turn and he won't take 20% damage that turn, but it will be right back the very next turn. You'll want some high level generics to help shoulder the weight, but overall this isn't a difficult scenario due to Valvatorez being Valvatorez.